In response to the current outbreak of SARS-CoV-2 and the urgency around establishing evidence-based IPAC approaches, we and others have hypothesized that the virucidal efficacy of commonly used microbicides against this emerging coronavirus should be predictable on the basis of the known susceptibility of enveloped viruses in general to chemical microbicides7-9,22. In this paper, we confirm the virucidal efficacy of a variety of formulated microbicidal actives against SARS-CoV-2 and a number of members of the Coronaviridae family (Bo-CoV, HCoV-229E, MERS-CoV, SARS-CoV, and MHV-1), indicating similar virucidal efficacy across members of the Coronaviridae. On the basis of these results, we predict that any potential future emerging coronaviruses or other emerging enveloped viruses also readily would be inactivated by these microbicides. The necessity for use of microbicides in IPAC for emerging viruses is informed by the routes of transmission of the viruses, the likelihood that they will be deposited on HITES, the expected duration of survival of the viruses on such HITES, and the frequency of recontamination of the HITES by infected persons.
The primary route of person-to-person transmission of SARS-CoV-2 is thought to involve respiratory droplets and aerosolsreviewed in 24-26, leading predominantly to a respiratory tract infection. Secondary (indirect) transmission of SARS-CoV-2 through contamination of HITES by droplets and respiratory aerosols or other patient secretions/excretions (bronchoalveolar fluid, sputum, mucus, blood, lacrimal fluid, semen, urine, and feces) also is thought to occur24-27. The indirect transmission pathway, envisioned as a patient’s bodily fluids-HITES-hands-mucous membrane nexus, is supported by experimental transmission studies in animal models28 and by the results of investigations of the contamination of HITES with SARS-CoV-2 RNA in healthcare settings25,29-31. The detection of infectious SARS-CoV-2 in patient feces32,33, together with the data on survival of SARS-CoV-2 in fecal suspensions20,21, suggest that a fecal/oral or fecal/respiratory route of transmission is possible. Zang et al.34, upon being unable to recover infectious SARS-CoV-2 from RNA-positive human fecal samples, have argued that the virus is rapidly inactivated by simulated human colonic fluid. This conclusion is not consistent, however, with the findings of Xiao et al.32 or Zhang et al.33, who were able to recover infectious SARS-CoV-2 from human fecesreviewed in 35, or with the reports of Liu et al.20 and Chan et al.21 that SARS-CoV-2 remains infectious for hours in human fecal suspensions. The conclusion is also not consistent with results obtained for other coronaviruses, such as SARS-CoV and MERS-CoV35. These routes of transmission could involve direct transmission or indirect transmission involving the patient’s bodily fluids-HITES-hands-mucous membrane nexus mentioned above. The U.S. CDC has stated that “transmission of novel coronavirus to persons from surfaces contaminated with the virus has not been documented”, but nevertheless has provided guidance on surface disinfection36.
The finding of SARS-CoV-2 RNA in untreated wastewater37 and sewage38, is suggestive of, but certainly not proof of, the possibility for survival of infectious virus within these human waste streamsreviewed in 35,39,40. Unfortunately, there are, to our knowledge, no data on the detection or persistence of infectious SARS-CoV-2 in wastewater, and this topic, therefore, remains a knowledge gap35,39,40. For the moment, on the basis of the reported survival of SARS-CoV-2 in human fecal suspensions and urine20,21, we assume the possibility of the contamination of wastewater streams with infectious SARS-CoV-2, and associated risk of virus dissemination through this route.
In order to inform the necessity of effective and frequent HITES decontamination during a virus pandemic, such as that being experienced currently with SARS-CoV-2, we have summarized the recent data on the survival of infectious SARS-CoV-2 on such surfaces under ambient conditions. Infectivity half-life values obtained from virus survival studies can be used to calculate the burden of infectious SARS-CoV-2 expected to remain on a surface after varying durations of time following initial virus deposit. This assumes, of course, that the initial virus load on the surface is known. There, unfortunately, is a paucity of empirical data on infectious SARS-CoV-2 burden (loads) on HITES in the literature thus far. The existing data consist primarily of measurements of nucleic acid burden on HITES. Findings from Matson et al.17 suggest that caution should be taken when making inferences regarding the possible presence of infectious virus on a surface, based solely on RT-PCR detection of viral RNA. We very much share this concern.
The data on the survival of SARS-CoV-2 on surfaces14-21,41, like previous data obtained for other coronaviruses41-49, demonstrate that viral persistence (survival) on HITES is dependent upon: 1) the type of surface, 2) the presence and type of organic matrix in which the virus is suspended at the time of deposition and drying upon the surface, and 3) time. The survival data for SARS-CoV-2 dried on surfaces (Table 1) indicate that the virus remains infectious longer on hard non-porous surfaces, such as plastic and stainless steel, than on wood or cardboard. The presence of an organic load during drying of the virus typically results in increased half-life of SARS-CoV-216,18. The result of Matson et al.17 that SARS-CoV-2 displayed a shorter half-life when dried on a surface in the presence of human sputum and mucus than when dried in a culture medium matrix was therefore unexpected, and requires confirmation. Temperature and relative humidity likely also play a role in the persistence of coronaviruses on HITES, although the data sets appearing in the literature specifically for SARS-CoV-214-21 have primarily evaluated survival under ambient conditions. For survival dependence on temperature, see references17,19,21,41.
The viral persistence data indicate that infectious virus may remain on non-porous HITES for one or two weeks. The risk of acquiring a SARS-CoV-2 infection indirectly, through transfer of virus from a contaminated HITES to a susceptible mucous membrane through the intermediacy of the hands, therefore may remain for weeks after the initial surface contamination event. The requirement for frequent sanitization of HITES is driven by the possibility for recontamination of these environmental surfaces by infected persons50. Infectivity data addressing the frequency of recontamination of HITES with SARS-CoV-2 have not yet appeared in the literature, to our knowledge. This represents another knowledge gap. In the case of human coronavirus 229E, Bonny et al.51 demonstrated that infectious virus could be recovered from HITES (desktops and door knobs) in a university classroom that was cleaned daily with a commercial cleaning solution consisting of non-ionic and anionic surfactants. This result suggested the possibility of the frequent recontamination of the HITES, although the possible inadequacy of the daily cleaning regimen was not ruled out by the authors51.
The stability of SARS-CoV-2 in suspensions and on skin has also been investigated. The survival of the virus in human sputum and mucus is similar to that on porous surfaces (half-lives of 1.9 to 3.5 hours, respectively)17. Survival on skin (3.5 hours)19 is similar (Table 1). These half-life values indicate that the virus may remain infectious for days following a contamination event, in the absence of hygiene interventions.
From the foregoing, it is apparent that there is a risk of indirect transmission of SARS-CoV-2 from contaminated HITES, via the intermediacy of hands. This risk may be mitigated through targeted hygiene interventions, including frequent surface hygiene as well as hand hygiene. The required frequency of surface hygiene interventions is dependent on the expected rate of recontamination of HITES by patients. This suggests that greater vigilance with respect to targeted hygiene practices is required in intensive care units and other contamination hot spots, as emphasized by Zhang52 and by the results of Wu et al.53
The required efficacy of targeted hygiene agents (formulated microbicidal actives) for reducing the infectious titer of SARS-CoV-2 and other coronaviruses depends, in large part, on the burden of infectious virus on the surface or in the suspension being sanitized54 and the human MID. Expected virucidal efficacy usually is expressed in terms of a minimal log10 reduction in viral titer to be achieved in standardized testing. For instance, the U.S. Environmental Protection Agency (EPA) specified in its 2012 disinfectant product guidance55 that “The product should demonstrate complete inactivation of the virus at all dilutions. If cytotoxicity is present, the virus control titer should be increased to demonstrate a ≥3 log10 reduction in viral titer beyond the cytotoxic level.” For disinfectants that are non-cytotoxic to the cellular infectivity assays used for demonstrating virucidal efficacy, a 4-log10 reduction in viral titer is considered to be effective. These EPA requirements have been revisited in the 2018 revision56. In the revised guidance, a valid test requires: 1) that at least 4.8 log10 of infectivity per carrier be recovered from the dried virus control film; 2) that a ≥3 log10 reduction in titer must be demonstrated in the presence or absence of cytotoxicity; 3) if cytotoxicity is present, at least a 3 log10 reduction in titer must be demonstrated beyond the cytotoxic level; and 4) that the cell controls be negative for infectivity. The revised guidance therefore does not require that an efficacious product demonstrate complete inactivation at all dilutions.
In our virucidal efficacy studies, a variety of formulated microbicidal actives displayed complete inactivation of the challenge coronaviruses (including SARS-CoV-2), with the maximum log10 reduction values achieved depending on the limitations of the assays (namely, the maximum titer of virus applied to the test and the cytotoxicity associated with the formulated microbicidal active). In any event, log10 reduction values of ≥3 to ≥6 were obtained after relatively short contact times (i.e., ≤5 min). These contact times are relevant for surface disinfection interventions and, notably, the contact times required for the hand hygiene agents evaluated (handwash agents and hand sanitizing gels) were ≤1 min. The active ingredients used in the formulated microbicidal agents evaluated in Tables 2-4 included lipid envelope-disrupting agents (ethanol, QAC, detergents, phenolics), protein- and capsid-denaturing agents (ethanol, phenolics, sodium hypochlorite, inorganic and organic acids), and genome-degrading agents (ethanol, sodium hypochlorite)57. Each of these types of microbicidal actives was expected, on the basis of the known susceptibility of pathogens to microbicides7-9,22,57 (Fig. 2), to display virucidal efficacy against lipid-enveloped viruses, including SARS-CoV-2 and other coronaviruses. This principle of the hierarchy of pathogen susceptibility has also been embraced by the U.S. Environmental Protection Agency58. Our efficacy data presented herein confirm this, and indicate that the virucidal activities are approximately equivalent for a variety of alpha- and beta-coronaviruses. In addition, reviews and empirical reports of the efficacy of microbicides against SARS-CoV-210,21,46,47 and other coronaviruses23,42,44-47,59 have confirmed the expected virucidal efficacy of a variety of microbicides against these viruses in surface disinfection studies. Efficacy of microbicides tested in suspension studies has been discussed in recent reviews and empirical reports of the efficacy of microbicides against SARS-CoV-210,14,21,47 and other coronaviruses42,44,46,47,59,60. These also have confirmed the expected virucidal efficacy of a variety of microbicides against these viruses in suspension.
Taken together, these results imply that similar virucidal efficacies will be displayed by such microbicides against future emerging coronaviruses, including non-functional mutational variants (isolates) of SARS-CoV-2 (it is important to note that, to date, there has been identified only a single strain of SARS-CoV-2)61. The virucidal efficacies would be expected22 to apply also to other emerging enveloped viruses, such as Ebola virus62,63, Lassa virus, Nipah virus, and influenza viruses such as the recently emerging G4 genotype H1N1 swine influenza virus64 and the variant influenza viruses (H1N1v, H3N2v, H1N2v) in humans65. The latter expectation is supported by our own unpublished data on influenza strains and by a recent literature review59. These are important conclusions, given that there is a likelihood of emergence of novel coronaviruses and other enveloped viruses in the future.
Figure 3. Heirarchy of susceptibility of pathogens to microbicidal active ingredients. Certain formulated microbicides may include combinations of active ingredients, resulting in synergistic virucidal efficacy greater than that displayed by the individual active ingredients (modified from Sattar, 20078).
A large diversity of alpha- and beta-coronaviruses currently circulate in bat reservoirs66. These include the alpha-coronavirus, swine acute diarrhea syndrome coronavirus, which caused large-scale pig die-offs in southern China, and is able to infect human cells in the laboratory67. They also include a substantial diversity of SARS-related coronaviruses that include the progenitor lineages of SARS-CoV and SARS-CoV-2, primarily carried by horseshoe bats (Rhinolophus spp.)68-73. SARS-CoV emerged in 2002 within urban live animal markets in Guangdong, where a range of animal species being held there, as well as animal vendors themselves, were infected74. While the exact route of SARS-CoV-2 spillover from bats to humans is uncertain, evidence strongly implicates a similar live animal market as a site where infections were amplified, and where SARS-CoV-2 was identified on contaminated surfaces68,75. Subsequent clusters of COVID-19 have been reported in a large seafood market in Beijing, perhaps as a direct result of contamination of cold surfaces used to prepare food76. Thus, the role of food animals, food preparation, and contaminated surfaces in the spillover of these bat coronaviruses suggests a key role for disinfecting surfaces to mitigate spillover or early spread of novel bat coronaviruses.
There is also evidence that bat coronaviruses are transmitted regularly to people in southeast Asia, without involvement of wildlife consumption. First, diverse behaviors that bring people into contact with wildlife have been reported in South China77,78. Secondly, 2.79% of people sampled from communities living close to a bat cave in Yunnan, China, where SARS coronaviruses have been reported, were serologically positive for bat coronavirus immunoglobulin G (IgG)79. Extrapolating to rural communities across Southeast Asia where similar bats exist, and given that SARS-CoV IgG had a half-life of 2-3 years in SARS survivors, it is likely that hundreds of thousands of people are infected by novel bat coronaviruses each year. Surface disinfection and personal hygiene using agents that are effective at inactivating coronaviruses may, therefore, be critical to the control of the current SARS-CoV-2 pandemic, and in reducing the risk of future coronavirus spillover events.
Assuming, for the purpose of argument, that SARS-CoV-2 is transmitted from person-to-person in part through the patient’s bodily fluids-HITES-hands-mucous membrane nexus, what evidence do we have that implementing surface and hand hygiene interventions will mitigate risk of disseminating SARS-CoV-2? It is clear that face touching is a frequent human behavior80, suggesting that the indirect route of transmission occurring through the intermediacy of the hands is relevant, and highlighting the need for strict implementation of hand hygiene. This especially the case when coming in contact with patients’ bodily fluids and when touching potentially contaminated HITES. Evidence has now been reported that disinfection can lead to reduction in dissemination of SARS-CoV-2 from infected persons to uninfected family members. For instance, Wang et al.4 reported that the daily use of chlorine- or ethanol-based disinfectants for household cleaning was 77% effective in reducing transmission of SARS-CoV-2 within the families investigated. Diarrhea as a symptom of the primary infected household member was also found to be a risk factor for transmission within families, informing the importance of sanitizing the toilets and the bathroom itself4.
The relatively high risk of the bathroom for deposition of SARS-CoV-2 from patients onto HITES was also highlighted in the study of Ding et al.81 In that study, frequency of sanitization of HITES was twice daily using a chlorine-releasing agent. Out of 107 surface samples and 46 air samples taken from a COVID-19 hospital ward, only seven surface samples (two door handles, one toilet seat, one toilet seat cover, one bathroom washbasin tap handle, one bathroom ceiling exhaust louver, and one bathroom door handle) and one air sample (a corridor air sample) were positive or weakly positive for SARS-CoV-2 RNA81.
Since it is not known yet whether infectious SARS-CoV-2 persists in wastewater streams39,40, we cannot address the question of whether hygiene interventions can reduce the infectious viral burden of such waste streams. This remains a significant knowledge gap that has yet to be closed82. There are data on the persistence of infectious virus in water for other coronaviruses, such as transmissible gastroenteritis virus, mouse hepatitis virus-1, and SARS-CoVreviewed in 40,41. For the moment, the use of wastewater/sewage SARS-CoV-2 RNA data is limited to a biomarker for monitoring of ongoing COVID-19 outbreak intensity37,38. It is evident from the foregoing discussion, however, that targeted surface/hand hygiene, appropriately practiced under healthcare, community and home settings, can help to ensure that infectious SARS-CoV-2 is not released into the environment via wastewater streams, and can potentially aid in IPAC of SARS-CoV-2.