SARS-CoV-2 Transmission Risk and Oral Decontamination: Scarce Evidence Albeit Promising Future*

Background: Oral decontamination recommendations/guidelines have exploded during the COVID-19 pandemic for the contemporary dental practice, due to SARS-CoV-2 relative high presence in saliva and the possibility of risk contagion through its aerosolization. However, such guidelines are mostly based on research carried out for other diseases caused by different viruses and/or bacteria, low-level evidence publications, and/or anecdotal information. Purpose: To review the biological basis for the use of oral antiseptics to decrease viral load in saliva as a plausible mechanism for reducing SARS-CoV-2 transmission risk, including other aspects such as pathogenesis, angiotensin converting enzyme 2 expression in the oral cavity, aerosolization, and oral antiseptics potential mechanistic virucidal properties. Results: Our group could only identify a limited number of reports evaluating specific direct effects of commonly used oral antiseptics (Hydrogen Peroxide, Povidone-Iodine and Chlorhexidine) on SARS-CoV-2, however, these reports are limited to surface disinfection, in vitro activity, or preliminary in vivo observations. Conclusion: Although we conclude that there is no direct evidence of clinical effectiveness of the use of mouth rinses prior to dental procedures with antiseptic solutions for SARS-CoV-2 specifically to date, we here present recommendations that could aid in reducing the risk of transmission in the dental office.


INTRODUCTION
The relevance of COVID-19 in the spectrum of infectious-contagious diseases is extremely high for the dental profession. Although the factual SARS-Cov-2 infectious potential is currently unknown, some important factors related to dental practice make it an important and sensitive issue: 1) The virus is apparently found in saliva at all stages of the disease (1)(2)(3)(4).
2) The probability of continued presence of SARS-CoV-2 in the oral cavity is very high due to its tropism to upper and lower respiratory tracts (5,6). 3) The contagion routes include drops and possibly aerosols as routes for their dissemination (7,8). 4) The epithelia of the oral cavity, the tongue and the ducts of the salivary glands highly express angiotensin converting enzyme "ACE2" receptors (although the receptor role in infection is not completely elucidated) (9)(10)(11)(12). 5) There is a large percentage of asymptomatic cases that could become a focus of transmission (13)(14)(15). Taken together, these factors have justifiably led the dental community to strongly consider preventive efforts in reducing risk to protect both clinicians and patients in the dental office environment.
The protocols published to date for dental care during the COVID-19 pandemic mostly include recommendations for the use of antiseptic solutions in rinses as an important strategy to lower the viral load in the oral cavity. However, when analyzing the scientific basis that supports the recommended solutions, one can find that they range from anecdotal recommendations to those based on studies carried out on viruses with either similarities to other coronaviruses (CoVs) or very different characteristics from SARS-CoV-2 such as non-enveloped viruses (16). Only some direct evidence on SARS-CoV-2 is available from in vitro studies (17) and some in vivo preliminary results. In this integrative review we aim for a two-fold objective: 1) to discuss the plausible biological basis for the use of oral antiseptics to decrease the viral load in saliva as a mechanism for preventing or reducing SARS-CoV-2 transmission risk and 2) to analyze current recommendations from the biological basis and then based on the chemical composition and antiviral activity of the different antiseptics available on the market. We also aim to establish criteria that allows the dentist to give adequate clinical instructions to their patients that could help decrease the virus transmission risk. We hope from here researchers could generate questions that serve as a starting point for new research that better supports clinical decision-making and worldwide preventive recommendations.

COVID-19: Knowns and Unknowns
Since the first report of COVID-19 made in December 2019 in Wuhan, China (15,18), the disease has spread throughout the world in such a way that the World Health Organization declared it pandemic on March 11, 2020, at which time more than 110,000 people were affected worldwide (19). The most common reported manifestations encompass fever, fatigue, and dry cough. Other reported symptoms include myalgia, chest tightness, dyspnea, nausea, vomiting, and diarrhea. The most common laboratory findings are lymphopenia or leukopenia. CT scans of the chest show typical images of viral pneumonia with multiple bilateral ground glass image opacities (20)(21)(22).

About 26 % of patients can present with anosmia and 22 % ageusia with a duration of about one
week (23), in addition to other neurotropic signs such as headache, vomiting, and confusion. About 20 % of cases require hospitalization (24). People with comorbidities prior to the development of obstructive pulmonary disease or who are complicated by secondary bacterial pneumonia are those who present a more serious clinical picture; some of the associated comorbidities are high blood pressure, diabetes, obesity, and chronic lung diseases, active smokers, and the elderly are also more susceptible (25). The so-called "cytokine storm," a systemic inflammatory syndrome that occurs due to hyperactivation of immune cells specifically leukocytes and endothelial cells that has been described in other diseases and is not exclusive to COVID-19, occurs more frequently in these comorbid patients, being one of the most important factors for higher mortality rates. Other mechanisms associated with mortality are multi-organ failure, acute respiratory disturbance, and disseminated intravascular coagulation (26).
SARS-CoV-2 infection has a high degree of infectivity during the incubation period, with rapid transmission even in young patients, in whom the disease develops rapidly and with manifestations that may be atypical, but with milder signs than in older patients. The median incubation time is 2 days with a range between 1-4 days in these patients (20). Additionally, a higher SARS-CoV-2 load is found in older adults, which could be associated with impaired immunity or with an increased expression of ACE2 (27). Noteworthy, a large number of asymptomatic undiagnosed cases likely end up affecting the real COVID-19 pandemic potential due to their particular epidemiological characteristics. Such cases are usually patients with mild or even non-existent symptoms, which is why a large number of people may be exposed to contagion through them (15). The actual frequency of asymptomatic cases as well as the time during which these cases can be reservoirs of the virus are currently unknown (28). The evidence that asymptomatic patients can transmit the disease is rapidly increasing (29,30). It is estimated that an asymptomatic patient could infect a number close to 100 individuals (13) and that transmission through them is responsible for 50-80 % of COVID-19 cases (14,15). It is also not known whether patients in the recovery phase might be possible transmitters (31).
The replication mutation rates of RNA viruses are higher than those of DNA viruses and the genomes of RNA viruses are generally less than 10 kb; however, the CoV genome is larger, at 30 kb approximately (37,38). The SARS-CoV-2 envelope is composed of a lipid bilayer that comes from the host cell membrane, in which four structural proteins (N, M, E and S) are coupled together with a set of nonstructural proteins. Protein N forms the nucleocapsid and its main function is the binding to the CoV genome, it participates in viral RNA replication and in host responses against infection. The M protein works in the virus assembly as it activates cell membranes to produce new viral particles. In several CoVs the M protein is in the vicinity of the Golgi apparatus. E is the smallest protein, it is widely expressed within the cell during the replication cycle, but only a small portion of it is incorporated into the virion envelope. Due to its location in the endoplasmic reticulum, Golgi apparatus and intermediate compartments, which are sites of high intracellular traffic, it is thought to be related to assembly and budding. Furthermore, it has been described that The envelope is a lipid bilayer to which four proteins are coupled: 1-Protein N, forms the nucleocapsid and its main function is the binding to the CoV genome, 2-Protein M, works in the virus assembly activating cell membranes to produce new viral particles, 3-Protein E, is widely expressed within the cell during the replication cycle and 4-Protein S (Spike), a class I fusion trimeric glycoprotein with different functional domains; the S2 subunit of Spike mediates the fusion of the virus and the cell membrane, while the S1 subunit is associated with receptor binding functions.
The Spyke (S) protein is a class I fusion trimeric glycoprotein, which undergoes a substantial structural rearrangement in its process of binding the viral membrane with the host cell membrane (40). Protein S in SARS-Cov-2 is different from that of other CoVs and the rapid spread of COVID-19 is possibly attributed to this (41). S has different functional domains, close to the amino terminus, S1, and the carboxy terminus, S2. The S2 subunit is a transmembrane protein that mediates the fusion of the virus and the cell membrane, while the S1 subunit is peripheral and is associated with receptor binding functions (39). After efficiently binding to the cell surface, the virus enters the cytosol using acid-dependent proteases, which disrupt protein S, followed by fusion of the virus cell membranes and the host cell. Spike is cleaved at 2 positions in the S2 domain of the protein; the first cleavage separates the 2 domains (RBD receptor-binding domains and the fusion domain) and the second, exposes the fusion peptides; the breaking leads to the detachment of S1 and to the re-bending (Refolding) of S2 (40) (figure 1). The fusion process occurs in endosomes. Once this happens, there are structural changes that allow the membranes to fuse, and the viral genome is released into the cytosol of the host cell. This occurs in the same way in SARS-CoV and SARS-CoV-2. The cleavage / fusion process is mediated by a protein called furin.
Furin clears the SARS-CoV-2 S protein at the S1 / S2 site, which is required for efficient cell / cell fusion (42).
It should be emphasized that the proteases expressed by the host cells decide the efficiency of the entry of the virus and thus its pathogenicity. In SARS-CoV-2 the S protein has 12 nucleotides extras that form sequences equal to the furin-like cleavage site, rich in arginine (42), which facilitates the priming of protein S and therefore the efficiency of the spread of SARS-CoV-2, compared to other CoVs (41). Additionally, changes in the S protein are responsible for variations in tissue tropism and the pathogenesis of the virus. On the other hand, S is the main target of neutralizing antibodies against SARS-CoV-2 (39,43). Studies have also shown that the pattern of expression of proteases in host cells may be an additional determinant in the tropism of SARS-CoV-2 for the cells it infects (43).

SARS-CoV-2 receptors. SARS-CoV-2 recognizes angiotensin-converting enzyme 2 (ACE2) and
it is through this enzyme that it binds to host cells, by a binding process dependent on serine proteases (TMPRSS2 and TMPRSS11D7) (12). ACE2 is a homologue of the angiotensin-converting enzyme (CE) in the renin-angiotensin-aldosterone system (RAAS), involved in the regulation of blood pressure and electrolytic homeostasis (44). Specific inhibitors of this enzyme are the most prescribed class of drugs in cardiology for the treatment of hypertension and heart failure. Some genetic variations in the receptor expression may be related to the existence of resistant populations (10). SARS-CoV-2 has a binding affinity for ACE2 10-20 times greater than that of SARS-CoV (40), which could partially explain the apparently easier transmissibility of the new CoV, further increasing the susceptibility of the host cells to its entry. Recently, three (3) additional SARS-CoV-2 receptors have been reported: 1) CD147, also known as Basigin, which is mainly expressed in erythrocytes, tumoral tissues, inflamed tissues, and pathogen-infected cells (45). 2) CD26, also known as DPP4, also found in epithelia and hematopoietic cell lines (46) that could explain cellular cross-reactivity with normal cells or facilitate virus entry in immature immune cells (47). 3) Neuroplin-1 (NRP1) a receptor involved in the development of the cardiovascular system and angiogenesis among others that enhances the infectivity of SARS-CoV-2 by binding substrates cleaved by furin (48).

SARS-CoV-2 via ACE2 entry into the body.
There is a widespread expression of ACE2 in healthy human organs. Receptor expression has been searched for by various methods, some of which only indicate the presence of transcripts or the DNA that encodes it and others the expression of the protein and its location in tissues, which could be more useful when explaining entry routes. Through the expression of mRNA for ACE2 it has been found in cardiovascular tissues, kidney, and testicles, as well as in tissues of the gastrointestinal system such as ileum, duodenum, jejunum, cecum, and colon. Furthermore, the expression of mRNA for ACE2, although limited in central nervous tissue, is positive in neurons and glial cells of the brain, so it could be thought that there is neuro invasion by the new CoV, which explains the neurotropic signs of COVID-19 (49). High ACE2 expression was found in endothelial cells of veins and arteries in all the tissues studied, as well as in associated smooth muscle cells, which would allow the virus to spread to multiple organs by this route.
However, since the virus has not been found in several organs, it could be assumed that vascular abnormalities and inflammatory changes could be related to the systemic cytotoxic effects of immune reactions caused by CoV infection. Similarly, this could indicate that as with HIV, CoVs (SARS) need a co-receptor molecule to enter cells (9). Considering that the pulmonary alveoli and the small intestine are organs that are related to the external environment, the expression of ACE2 in the epithelial cells of these 2 tissue types postulates them as potential entry routes for SARS-CoV-2. SARS-CoV-2 then could initially enter through the mucous membrane of the upper respiratory tract, especially the nasal or pharyngeal epithelium, or directly into the lower respiratory tract and infect the bronchial and alveolar epithelial cells from there. Once this phase has started, the virus can pass from the lung potentially into the blood and initiate a viremia, reaching other organs that express ACE2. The intestine could also be reached orally (10) (figure 2).  Figure 2. The entry of SARS-CoV-2 into the human body depends on the interaction with receptors and enzymes on the surface of host cells. The virus primarily binds to ACE2 in the presence of serine proteases (TMPRSS2). The cleavage process of the S protein and its fusion with the host cell is mediated by the Furin protein, whose role seems to be important in the greater spread efficiency of SARS-CoV-2 in relation to other CoVs. The expression of CD147 and CD26 on the erythrocyte mediates virus infection as well as its possible entry into immature immune cells. ACE2 is expressed mainly in the epithelium of the pulmonary alveoli. Despite their expression in multiple tissues, it is not certain whether all of them represent routes of infection. For example, in the epithelium of the oral and nasal cavity as well as in the epidermis, ACE2 is expressed in the basal layer and not in the superficial layers, so its relevance in the initial infection is debatable, especially through aerosols.

ACE2 in the Oral Cavity
Using analysis of databases of RNA sequences from normal tissues and of tumor origin, it was determined in the oral cavity that 95.86 % of ACE2-positive cells are found in the tongue, above the oral mucosa and gingival tissues (11). In Rhesus macaque, high expression has been demonstrated in cells morphologically compatible with epithelial cells from the ducts of the salivary glands (50). Such high expression of ACE2 mRNA has also been reported for minor salivary glands, being even higher than in the lung (51) as well as the expression of marginal levels of TMPRSS2 mRNA in cells. Epithelial of the salivary glands can lead us to think that the salivary glands could be a site of entry for SARS-CoV-2 (12).

Presence and Viral Load of SARS-CoV-2 in Saliva
Several factors can be associated with viral load, especially the time after infection and the stages of the disease. The average duration of viral spread has been estimated at 24.5 days and appears to be independent of the clinical manifestations of the patient, but the duration is longer in symptomatic patients (25.2 days) vs. asymptomatic patients (22.6 days), especially those with chest pain and sputum production (23). Saliva may be a source of COVID-19 transmission among humans, but the study of the viral load in saliva is not only related to the routes of transmission, but also to the development of less invasive diagnostic tests that are easier to perform under safer conditions (2). In saliva, the virus can also be studied in its active replicative state, which is the most possible form of transmission. Given the accessibility of saliva, the role of the molecular characteristics of the virus and the potential for genetic changes that are related to high transmissibility and that may be possible therapeutic targets could be studied (1).
Three routes are proposed by which the virus could reach saliva: 1) The virus arrives from the respiratory tract by fluids that are exchanged with these organs. 2) From the blood the virus reaches the gingival crevicular fluid. 3) Due to infection of major and minor salivary glands it is released into saliva through their ducts (52). The primary evidence of saliva as a transmission route has been obtained from studies carried out with other CoVs given their similarities. One of the most cited is the one performed on Rhesus Macaque monkeys, in which the virus was found in the oral swabs of 4/4 animals vs. 2/4 with infection in the lung, suggesting an increase in viral reproduction in the upper respiratory tract, including salivary gland ducts, in early stages of the disease. This study also suggests that the lining epithelial cells of the salivary gland ducts are a source of virions found in saliva droplets in these early stages. Similarly, viral mRNA has been found in saliva taken directly from the opening hole of the salivary gland ducts, confirming the possibility that the salivary glands are a site of replication and that the virus is released directly into saliva (21).
However, it is important to note that there is a possibility that virions found in saliva come from other sources, such as respiratory secretions, as viral load in sputum is significantly greater than in the oropharynx and nasopharynx especially in late stages of infection (50,53).
Greater viral loads have been reported in the nose than in the throat; however, in both sites it has found to be equally high immediately after symptoms onset. Viral load can also be similar in symptomatic and asymptomatic and indicates greater potential for transmission in days after infection. Nevertheless, it is important to note there is no correlation between virus positive qPCR and presence of cultivable virus; the latter could be cases where there may be mild or non-existent symptoms with very modest levels of detectable viral RNA in the oropharynx for at least 5 days (6).
The correlation of the detection of SARS-CoV-2 in serial samples of saliva with samples of nasopharyngeal swaps is high. As recently reported, the sensitivity of saliva tests can reach 87 % (95 % CI, 65-97 %) (1). In serial samples of expectorated saliva from 12 patients diagnosed with COVID-19 (qPCR and culture), virus detection in saliva was achieved in 11/12 specimens (91.7 %). Conversely, patients with negative nasopharyngeal isolates showed saliva to also be negative.
Therefore, saliva samples have been recommended for monitoring as they are easier to take (54).
A recent meta-analysis reports a sensitivity of these tests of 91 % vs. 98 % of the tests with nasopharyngeal isolate samples in patients previously confirmed with COVID-19 (55).
Despite the apparent utility of saliva as a sample for the detection of SARS-CoV-2 and the diagnosis of COVID-19 there are still many gaps. The forms of saliva collection, its handling and pre-test processing, and other variables with reference to its manipulation in the laboratory must be standardized before making an accurate interpretation of the biomarkers in saliva related to the infection by SARS-CoV-2. It is necessary to determine the viral load in saliva of asymptomatic and pre-symptomatic patients. The great advantage is that in saliva analytes that go beyond the presence of the virus can be identified (56).
To date, data on the transmission of SARS-CoV-2 by blood or serum are scarce. Although the presence of virus mRNA in blood has been demonstrated in percentages as low as 1 % of samples (57), there are no data on the viral load in these fluids or lymphocytes in the different periods of infection, specifically during the incubation period. Thus, it cannot be ensured that there is a risk of contagion by blood either by contact or by transfusions (58). There is also no published evidence of the presence of SARS-CoV-2 in crevicular fluid, although it could be a possibility that is hypothetical and speculative at this time (59). Given the low percentages of viremia in the blood, speculation that the virus may pass from the blood into the crevicular fluid and from there into saliva seems rather unlikely.

Transmission Routes in the Dental Practice
Given the low amount of evidence, it is essential that all possible routes of transmission are considered so that all prevention fronts are covered. Transmission routes can include upper and lower airway secretions and saliva (both airborne through aerosols and by contact with contaminated surfaces) as well as fecal-oral transmission (27), which has also been considered a plausible route. Our review will focus on aerosol transmission.
Coughing and sneezing produce ejections that are multiphase turbulent clouds containing hot humid air, and suspended droplets of mucosalivar fluid, which may contain pathogens (60). The contamination range of the droplets is largely determined by its size. The physical mechanism of droplet formation at the outlet of the mouth after sneezing, analyzed by high-speed imaging, reflects that the breakdown of the droplet fluid continues outside the respiratory tract during violent exhalations. This includes a complex cascade of events ranging from leaf formation, to bag bursts, to ligaments, which eventually break into drops. The viscoelasticity of the mucosalivar fluid plays an important role in the delay of the fragmentation causing the combination of the precursors of the drops that form along stretched ligaments, which affects the final size distribution of the drops (61).
Historically, the study of aerosol generation in dental practice has been a priority topic. These studies have generated evidence of the presence of viruses such as HIV, hepatitis B, influenza, and herpes viruses in small aerosolized particles (62). During respiration, it has been found that drops containing microorganisms and viruses such as influenza can also be produced, hence it is thought that SARS-CoV-2 can be transmitted by coughing, sneezing, breathing and even when speaking (63).
In SARS-CoV-2 positive hospitalized patients, medical procedures such as bag valve-mask ventilation, non-invasive ventilation, and intubation can create localized aerosols that could allow this type of transmission to those involved in the maneuver. Given the low knowledge in this regard, the Public Health Agency of Canada guidelines requests that suspected or confirmed patients be placed in airborne isolation (negative pressure and constant air change) and, if not available, in a room alone behind closed doors (64). Large droplets can infect nearby subjects and small droplets containing viral particles suspended in the air, could be transmitted over larger distances (52). The real potential for large droplets to deposit on surfaces or impact the face or eyes of a subject is not very clear; it has been estimated that drops with a diameter greater than or equal to 100 μm tend to fall rapidly and travel short distances (not more than 2 meters) before evaporating, while the smallest of less than 20 μm evaporate and become "droplet nuclei," remaining in the air for much longer being able to travel distances greater than 2 meters (65,66).
The drops that are emitted when speaking are smaller than those emitted when coughing or sneezing, but they are large enough to carry pathogens. Furthermore, it has been shown that talking can produce more drops than coughing (14). Talking hard emits thousands of drops of oral fluid per second, which if they are in a stagnant microenvironment have a decay time ranging from 8-14 min, which would correspond to a nuclei droplet of ca 4 μm in diameter, or 12-21 μm before dehydration (67). Those speaking drops generated by asymptomatic SARS-CoV-2 carriers could be considered possible routes of transmission, since they can generate airborne virus transmission in closed environments.
Despite the controversies over the potential for transmission by aerosols, today it is accepted that SARS-CoV-2 remains with the potential of infection for hours in aerosol drops, although it is clarified that it is necessary to know the viability of the viral particles present there (8). The physical characteristics of the droplets have dominated the discussion about their transmission in air, but the chemical characteristics of the droplets have usually been neglected, despite the fact that this microenvironment in the droplet is an important determinant of their stability. For example, the viability of the influenza virus is inversely correlated with the concentration of salt in evaporated drops that contain few proteins (68). When the drops are expelled from the respiratory system, they undergo changes due to environmental conditions. Except in very humid environments, the water in the drops evaporates quickly, so the concentration of salts, proteins and other components increases. The pH also influences the change of the drops. These changes have important implications for the viability of any pathogen that is inside them and therefore can affect the efficiency of the transmission of infectious diseases by drops and aerosols (69).

Can Oral Decontamination Reduce SARS-CoV-2 Transmission Risk?
Concerns about aerosol production-dependent contamination in dental practice has existed for a long time (70) and significantly regained in the face of the new pandemic. Even though to date no documented transmission of SARS-CoV-2 by aerosols in dental practice has been reported yet, aerosols generated in the dental practice can create a highly polluted environment conducive to transmission (31). Aerosolized saliva exposure peak for dentists/assistants has been estimated to range from 0.014-0.12 µL at 15 minutes (71). Furthermore, aerosolized microorganisms in dental offices can remain in suspension for up to 4 hours after dental procedures, so staff can be exposed at the time protective equipment is removed. Therefore, reducing the burden of microorganisms in the oral cavity seeking aerosolization reduction and cross contamination reduction becomes instrumental (72).
Dentists are classified by the Occupational Safety and Health Act (OSHA) as high risk exposure workers to known or suspected COVID-19 positive patients (73). According to OSHA recommendations in the context of the global pandemic, dental procedures involving the generation of aerosols should be fully avoided. Due to the impact of the pandemic on humanity and the ease of transmission and dissemination of COVID-19, panic has been generated among the population. Dentists around the world, who despite having extensive biosafety knowledge/experience in their professional practice, are in a state of anxiety and fear while working in patients; this has led to modifying their services in accordance with the guidelines or treatment protocols for patients who attend dental consultation during the COVID-19 pandemic (74). Dentists then ought to implement prevention strategies to reduce SARS-CoV-2 transmission risk by focusing on disinfection of the workplace, exhaustive hand hygiene, proper personal protective equipment (PPE) use, and importantly aiming to reduce virus aerosolization. Oral decontamination then becomes a key strategy in pursuing the latter.
Nevertheless, the biggest limitation with oral decontamination protocols is the lack of proper evidence documenting a real virucidal activity the clinician can count on. To date, there is no evidence of clinical effectiveness of the use of mouth rinses prior to dental procedures with antiseptic solutions for SARS-CoV-2 specifically (75). As the dental profession adapts to the new challenges, and more applicable research is procured, here we will review the composition and function of the most common oral decontamination products aiming to reduce transmission risk.
The most common oral antiseptics aiming for COVID-19 inactivation are listed in Table 1. In vitro on surfaces: PVP-I at 0,5 % 1 % and 1,5 % completely inactivate SARS-CoV-2 USA-WA1/2020 strain within 15 seconds of contact (80). Routine administration of PVP-I indicated mainly in symptomatic patients infected by SARS-CoV-2, especially during the first week after the onset of symptoms when viral loads in saliva are higher (82).
-Gargling before, during and after induction and stabilization of hospitalized patients (87).

Hydrogen Peroxide (H202)
Oxygenated agents such as hydrogen peroxide (H2O2), buffered sodium peroxyborate, and peroxycarbonate, are recommended for short-term use as disinfectants. H2O2 in particular, can be seen as a natural disinfectant because after use it rapidly breaks down into non-toxic products (water and oxygen). It was first synthesized in 1818 by Louis Jacques Thénard, by reaction between nitric acid and barium peroxide, but currently it is produced from anthra-hydroquinone when it reacts with oxygen under pressure, which generates hydrogen peroxide and anthraquinone. Hydrogen peroxide is a light blue liquid that when diluted appears colorless; it is soluble in water and is composed of hydrogen and oxygen (88). H2O2 oxidizes when in contact with organic matter, metals and alkaline solutions by producing hydroxyl free radicals that react with lipids and proteins. H2O2 has also been found to cause DNA damage by oxidation, induced by reactive oxygen species (ROS) that are released when hydrogen peroxide is degraded. Oxygen free radicals are molecules with unpaired electrons that contain oxygen. Hydrogen peroxide is normally reduced to water (H2O), but some metal ions such as iron, copper and titanium can contribute to the production of highly reactive hydroxyl free radicals (OH-) which can damage tissues (89) ( figure 3A). An important characteristic of hydrogen peroxide is its high instability, as it can be degraded by catalysis, exposure to light, movement and temperature.
In terms of antiseptic properties, H202 is considered broad-spectrum for gram-positive, gramnegative, sporulated bacteria, yeasts and viruses. The variable levels of catalase and other peroxidases in the microorganisms explain the various degrees of tolerance that can be presented to this agent, especially at low concentrations (3 % to 6 %) (90). Therefore, the concentration at which H202 gets used determines its real antiseptic effect, being bactericidal between 3 and 6 %.
However, at this concentration H202 has limited activity on spores and requires longer contact times. In contrast, high concentration solutions (10 % to 30 %) have shown rapid in vitro effect on spores (91).
Viruses such as HCoV, MERS, and SARS have been found to persist on inanimate metal, glass, and plastic surfaces for more than 9 days, and can be effectively inactivated by disinfection procedures with agents such as ethanol between 62 % and 71 %, sodium hypochlorite, and 0.5 % hydrogen peroxide, among others, it is considered that its use may be key in preventing the spread of SARS-CoV-2, especially considering that since it is a new infection no specific therapy is available (92). Some studies have found that H202 is effective for the inactivation of CoVs such as the CCV strain I-71, HCoV strain 229E and SARS-VOC isolation FFM-1 on inanimate surfaces, when tested at concentrations that are between 0.001 % and 1 %, in a dose-dependent manner, with exposure times between 1 and 10 minutes, where the lowest concentrations are those that require the longest exposure time (93)(94)(95). The inactivation of SARSCoV-2 on inanimate surfaces with the use of hydrogen peroxide has led to increased use during the pandemic in efforts to reduce hospitalization times (78). The latter report proposes that not only H202 may be beneficial due to its oxidation properties, but also indirectly through innate immune antiviral responses via TLR3 overexpression (96) potentially reducing the progression of infection to the lower respiratory tract.
Consequently, the use of 3 % H2O2 has been recommended for performing nasal and oral lavages in patients who present the first symptoms of SARSCoV-2 infection, in whom the diagnosis has already been made. Also, for patients who already have frank disease symptoms who are quarantined at home and/or hospitalized patients who do not need intensive care (78). The suggested protocol for mouthwash is to be carried out 3 times a day, while for the nasal mucosa exposure should be carried out by nebulization 2 times a day only because of its irritant effects.
This therapeutic scheme still needs further evidence for adoption.
An in vitro study determined the antiviral efficacy of eight commercially available oral rinses based on different active compounds (exact formulations for these oral rinses were not publicly available due to patent-related restrictions), were tested on 4 isolates of SARS-CoV-2, under conditions mimicking nasopharyngeal secretions. The virucidal activity was determined with a quantitative suspension test for 30s exposure time. For 1.5 % stabilized hydrogen peroxide, a log reduction factor between 0,33 % -0,78 % for 3 strains was demonstrated, achieving a reduction which was not better than other tested compounds (77).
Another in vitro study made on SARS-CoV-2 USA-WA1/2020 strain cultures testing a Thus, Preventive iodine therapy should be done within safety limits and taking into account contraindications for its use. Nevertheless, this intervention is a cost effective approach, with low risk, and potentially easy to develop and adopt on a global scale.

Hypochlorous Acid / Saline (HClO)
Hypochlorous acid (HClO) is also known as chloric acid (l), chlorinol, hydrogen chlorate, hydrogen hypochlorite, chlorine hydroxide, electrolyzed water, electrolyzed water in oxidation, and electro-activated water. Physiologically, HClO is naturally produced on cells of innate immunity by a chain of oxygen-dependent reactions known as respiratory burst, the purpose of which is to kill invading pathogens and control infection. The most abundant anion in humans is chloride (Cl -), which is necessary for the function of the innate immune system cells. Resting polymorphonuclear cells have been reported to have a high concentration of Cl -, which is mobilized through the hydrophobic lipid membrane of cells, and whose exchange by enzymatic interactions, mediate the formation of HClO inside the phagosomes (111). Phagocytes can also destroy ingested microorganisms by ROS production during the respiratory burst including HClO, via myeloperoxidase. An activated neutrophil produces around 1.6 x 10 6 HClO molecules / second and in phagosomes between 28 % and 72 % of the consumed oxygen is converted to HClO; therefore, a continuous supply of chlorine is required for HClO to be produced (112).
The highly destructive power of HClO as a non-selective oxidizing agent has been attributed to its ability to oxidize nucleotides, activate latent enzymes as well as the electron transport system, breaking of both cell membranes and basement membranes and fragment proteins (113).
Therefore, it is described as an oxygen-dependent, highly unstable, reactive and oxidizing nondissociated ion from chlorine, and therefore directly responsible for the rapid, chemotactic and broad-spectrum bactericidal action of chlorine-derived compounds (114) (figure 3A).
HClO solutions are not very stable due to various environmental factors, especially the presence of organic compounds and inorganic ions that result in a rapid consumption of it by oxidation reactions. Industrially, HClO results from the union of the acidic chlorine oxide with water and therefore the producer must use pure water preferably from a water purification system, with the lowest possible content of inorganic compounds and ions (115). It can be generated by electrolysis of sodium chloride (NaCl) and water (H2O); at pH 5-6, the chlorine species are almost 100 % HClO, but when the pH drops below 5, it begins to convert to Cl2 (chlorine gas). Above a pH level of 6, it begins to convert to a hypochlorite ion (OCl-).
HClO has a well-established antibacterial and antiviral effect, with greater potency than hydrogen peroxide (116). This effect has been demonstrated in encapsulated and non-encapsulated viruses, in DNA viruses such as herpes simplex virus-1, and RNA viruses such as respiratory syncytial virus, influenza A virus, and human CoV 229E (117). Interestingly, the mechanism for HClO production from a continued supply of NaCl constitutes the basis for the recommendation to use nasal washes and gargles with hypertonic saline in 1.5 % and 3 %; the latter under the assumption that the cells of the Nasal, pharyngeal and oral mucosa can convert NaCl to HClO ergo exerting an antiviral effect (118). In fact, with the use of saline solution, clinical improvements have been evidenced in patients who initiate viral diseases such as influenza (119), reflected in less need for drug consumption, fewer days of disease duration, lower rates of transmission, less viral elimination and less fever development (120,121). However, the evidence needs improvement and therefore the development of randomized controlled clinical trials is expected.

Cetylpyridinium Chloride (CPC)
CPC is a cationic quaternary ammonium compound; this amphoteric surfactant generally contains a quaternary nitrogen, associated with at least one hydrophobic substituent. Their cationic nature allows them to interact with the cell wall and membrane, displacing the bivalent cations Mg2 + and Ca2 + (122), altering the lipid bilayer, which generates the exit of the cytoplasmic components, killing the cell by solubilization of the membranes (123). The stabilization process by bivalent cations occurs in gram-positive and gram-negative bacteria, which also gives it a broad antibacterial spectrum. In addition to its direct microbicidal capacity, CPC has been reported to modulate signaling events associated with cellular transcription of nuclear factor kappa-B (NF-kB), a mechanism used by viruses to increase gene transcription and therefore viral replication The virucidal action of CPC is very likely to be just as effective on enveloped and non-enveloped viruses; its ability to kill enveloped viruses gives it a wide spectrum of activity. It was found to rapidly damage virus membranes within minutes after challenge, inhibiting its ability to infect.
CPC formulations reduce influenza-associated morbidity and mortality in vivo in mice (125).
Topical application of the dual-action formulation known as ARMS-I (Halo™) containing CPC, glycerin, and xanthan gum, used for the prevention and treatment of influenza, has been associated with a trend toward reduced severity and the duration of cough and throat discomfort (126). CPC can also inhibit HBV assembly by interacting with the viral nucleocapsid dimeric viral protein.
Additionally, it produces a significant decrease of HVB in cell cultures and an inhibition of its replication in mouse hydrodynamic modeling systems. It has also been shown to be active against poliovirus I (127).
Quaternary ammonium compounds have been used on several occasions for the treatment of several CoVs. Its action in the deactivation of the enveloped virus lipid cover suggests that it may be active against SARS-CoV-2. The real question is whether compounds with well-documented in vitro antiviral activity behave similarly in vivo (e.g., destroying the virus capsid and accumulating in lysosomes or endosomes blocking viral entry). If so, CPC is a good option to use as it is cheap, easy to obtain in hospitals and readily accessible for the consumer. Products such as shampoo and soaps containing CPC could aid in potentially helping to reduce viral spread (128). Chlorhexidine is probably the gold standard for antiseptic oral decontamination. Its most widely used form is digluconate chlorhexidine. Concentrations range from 0.003 % in some oral rinses to 4 % in surgical soaps. It has been incorporated into multiple hygiene products, especially at the hospital level. It is found in hand and skin cleansing formulations, for cleaning infected wounds, in surgical cements, topical antiseptics, oral rinses and gels and attached to slow release vehicles.
Its use as an antiplaque agent and therefore with effects on gingival inflammation has been extensively documented (131). As an oral antiseptic, CHX has a greater substantivity than other products. Once a rinse is performed, approximately 30 % of the compound is retained in the oral cavity through electrostatic interactions with the acidic groups on the macromolecules of the mucous secretions that cover the oral surfaces. The sequential displacement of chlorhexidine bound to the oral mucosa caused by bivalent cations such as calcium from saliva is suggestive of the slow release mechanism, up to 8-12 hours (132).
A limited viral inactivation potential/virucidal effect have been described with the use of chlorhexidine and other phenolic disinfectants (133). Based on this observation, multiple protocols for the treatment of SARS-CoV-2 mostly rule out CHX as a useful active ingredient for disinfection. To date, no publication has been found to prove or disprove its effectiveness; however, there is available data on other viruses with similar characteristics, specifically with the presence of viral envelopes. For example, the in vitro antiviral efficacy of 0. 12  Based on the principle of potential efficacy of CHX in enveloped viruses, a preoperative protocol has been proposed for optimizing infection control in the operating room during the COVID-19 pandemic.
It includes the use of chlorhexidine in both wipes and gargles for hospitalized patients (87). The study by Xu et al. (79), after an infectivity in vitro test and a cytotoxicity assay, reported that  (84). Although promising, the results of the present study must be carefully evaluated due to sample size limitations and study design. The -R groups of the amino acids of viral proteins: -SH (thiol), -OH (hydroxyl) and -NH2, -NRH or -NR2 (amino), which are not involved in the covalent bonds of the primary structure, are susceptible to oxidation by agents such as H2O2 (hydrogen peroxide), HClO (hypochlorous acid) and C6H9I2NO (povidone iodine). Neighboring -SH groups oxidize to -S-S-and -NH2 to -NO2. This leads to changes in the tertiary and quaternary structures of proteins, causing rupture of the membrane and alteration of function. B. C21H38ClN (Cetylpyridinium chloride) and C22H30Cl2N10 (chlorhexidine), due to their cationic and anionic nature, produce positive (C21H38ClN) or negative (C22H30Cl2N10) charges displacement present in the viral membrane. In the case of C21H38ClN, in addition to the displacement of cations, the negative charges of -COOgroups of the proteins are neutralized. In both cases, the alteration of the viral membrane leads to its rupture.

CONCLUSIONS
Dentists have been placed in a historically important situation with the current pandemic and its transmission risks, and as such dentists need to be informed with the best available evidence for the sake of their patients as well as their occupational health and safety. This narrative review has covered general aspects of COVID-19 physiopathology including SARS-CoV-2 characteristics, human receptors mediating cellular entry, disease evolution and viral loads in the oral cavity. As saliva emerges as a very plausible transmission route in asymptomatic patients who are not aware of their infectious potential, strategies to pursue transmission risk reduction ought to be pursued.
When considering the different pathways from which SARS-CoV-2 can reach the oral cavity and colonize the saliva, it is important to note that other respiratory fluids (sputum expectoration) can also saturate the oral pharyngeal region; therefore, gargling emerges as a better strategy when compared to only oral decontamination via mouth washing. Saline washes of the nasal passage, mouth, and throat could probably reduce viral load in the body mechanically (at least) in the initial stage of pathogenesis. This could be similar to hand washing to contain the spread of the infection.
Therefore, antiseptic gargles and nasal wash may work in preventing the disease and may also be useful in reducing nasopharyngeal viral load to provide symptomatic relief. Furthermore, it may reduce viral shedding and reduce the transmission of the illness. This may break the chain of infection. COVID-19 disease is mild in eighty percent of patients and resolves spontaneously.
Therefore, nasopharyngeal wash may be useful, especially in high risk populations such as subjects with comorbid conditions and above 60 years of age.
This narrative review has also covered the best available evidence from the most common oral decontamination agents readily available for the clinician. We can conclude that there is no highquality direct evidence of clinical effectiveness of the use of mouth rinses prior to dental procedures with antiseptic solutions for SARS-CoV-2 specifically to date. However, absence of direct evidence should not mean that nothing works on the virus, rather it should just mean that there is a lack of evidence for the moment. When indirect evidence is considered, particularly scarce on SARS-CoV-2 effects, there are multiple reasons to justify oral decontamination protocols with antiseptic agents. Although there is not sufficient evidence to support the use of one specific agent over the other, the antiviral capacity of all agents here reviewed is sufficiently suggestive of its potential SARS-CoV-2 virucidal capacity. Ergo, oral decontamination should be promoted in our opinion.
In sum, the possibility of completely eradicating SARS-CoV-2 is practically impossible. It is therefore necessary to learn how to establish a coexistence with this virus from different areas, being the dental one of those that requires an important degree of analysis. Current clinical practice is adapted through gradual changes that respond to the level of knowledge available in a dynamic process, in which protocols for clinical care, issued by academic and government organizations, have become very important. However, the dental profession has to quickly adopt such recommendations; surprisingly, a recent survey report from 669 dentists (30 different countries) found that 74 % of dentists (from which 76 % are in hospital settings) did not ask patients to rinse the mouth with antibacterial mouthwash before dental treatment (74). Our well-known biosafety practices in the dental office could have a greater impact in patient behavior and transmission risk reduction.
Finally, although it is clear that oral antiseptics could decrease viral load, the infectivity of the SARS-CoV 2 virus from saliva should be investigated both before and after rinsing with antiseptics, since these compounds act by superficial contact with the virus, but replication within cells is not affected; therefore, it is crucial to determine how long it would take for the virus to regain infectious potential in saliva. As of today, we can affirm that it is feasible to reduce the viral load in saliva with this type of strategy, but not that said decrease in viral load is an accurate preventive strategy in controlling the spread of the infection and even less use it as a therapeutic strategy. The fact that SARS-CoV-2 is fatal in a significant percentage of cases makes infectivity studies in vivo ethically impossible. In addition, preventive strategies should extend to trying to block the different viral proteins to prevent the virus from entering the cells in a way that combats viral replication, directly avoiding infection.