Relevance of mask-fit in the controversy of SARS-CoV-2 transmission
The predominant route of SARS-CoV-2 transmission, droplets versus aerosols, is controversial among aerosol scientists and the World Health Organization
On 11 March 2020, the World Health Organization (WHO) declared the illness caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) a pandemic. More than a year after, WHO and many scientists (aerosol experts) still disagree on the primary mode of transmission of this new disease, COVID-19 (coronavirus disease 2019). This disagreement is not a minor controversy, since WHO plays a leading role in the global policy response to COVID-19 as a mediator between science and policy. WHO advice influences both allocation of resources and management of the pandemic.
Many scientists claim that SARS-CoV-2 is transmitted primarily by infectious aerosols. Since these tiny particles can remain suspended in the air for hours and move long distances, control measures should include “ventilation, air filtration, reducing crowding and time spent indoors, use of masks whenever indoors, attention to mask quality and fit, and higher-grade protection for healthcare staff and front-line workers”. Although WHO agrees that airborne transmission of COVID-19 is plausible in some specific situations, it states that the lack of standardized guidelines for conducting and reporting research on airborne transmission prevents firm conclusions.
According to the WHO, SARS-CoV-2 spreads predominantly through virus-containing droplets emitted by the infected person and contact routes. In this case, the critical control measures are “reducing direct contact, cleaning surfaces, physical barriers, physical distancing, use of masks within droplet distance, respiratory hygiene, and wearing high-grade protection only for so-called aerosol-generating healthcare procedures”.
In some studies, health care workers (HCWs) were not infected despite not wearing respirators but surgical masks and other contact and droplet precautions. For WHO, this finding is an indication that aerosols are not the predominant route of COVID-19 transmission. Its rationale is as follows: if a) surgical masks and respirators provide the same protection in health care settings, and b) the main difference between surgical masks and respirators is fit requirements, then mask fit is unnecessary to prevent COVID-19 transmission.
Evidence provided by WHO contrasts with some reports in which HCWs wearing surgical masks became infected with SARS-CoV-2, despite not being involved in aerosol-generating procedures
The evidence supporting surgical mask usage for the prevention of COVID-19 in health care settings is currently inconclusive
Two randomized controlled trials with other respiratory viruses support surgical masks as a good prevention strategy in health care settings for standard procedures. In these studies, the incidence of laboratory-confirmed influenza in nurses wearing either surgical masks or N95 respirators were comparable. Some systematic reviews and meta-analysis also support this finding.
In contrast, two other randomized controlled trials support the superiority of N95 respirators in protecting HCWs against respiratory viruses.
In a recent study of HCWs in Finland, occupational COVID-19 infections occurred in those workers using surgical masks but not in those wearing FFP2/3 respirators and following aerosol-prevention guidelines. The authors concluded that surgical masks are not adequate protection against SARS-CoV-2 and that the use of FFP2/3 respirators in all patient contacts is recommended.
The difficulty in achieving a correct mask fit may act as a confounder behind these contradictory results
Mask fit has been neglected throughout the pandemic because regulators have not considered the predominant role of aerosols in the transmission of SARS-CoV-2. Manufacturers and regulators pay more attention to the filter material than to the fit.
The lack of control of training and adherence to medical masks and respirators may have reduced the differences in effectiveness between both type of face masks. Users do not always fit their respirators appropriately. In subjects performing conventional fit test exercises, the amount of air passing through the edges of N95 masks was up to 20 times greater than that passed through the filter. In the case of surgical masks, this amount was six times higher. Leaks were higher for infectious aerosol-sized particles.
Recently, a comparison of the efficacy of several N95s and KN95s (the Chinese standard for respirators) and a surgical mask showed a high proportion of fit test failures, resulting in reduced effectiveness. Four of the seven participants were unable to fit any of the N95 respirators tested correctly. The authors also showed that performing a self-test is an unreliable way to determine the fit. Even a tiny fit problem, not detected by the user during a self-test, reduced mask protection. Moreover, minor anatomical variations that may appear insignificant, such as the amount of subcutaneous fat under the chin, were found to impact fit significantly.
An optimal mask fit is difficult to achieve, since the face is not a static surface; for example, the jaw’s movement when swallowing, grimacing, or speaking is likely to disrupt the seal. Leaks increase the faster we breathe, as when we exercise. An excellent fit is essential to protect the user against aerosols. Even small leaks greatly reduce filtration efficiency. Leaks representing only 0.5% to 2% of the total mask area reduce total filtration efficiency by half to two-thirds.
Face mask protection also depends on comfort and breathability (the ease with which air passes through the material). An excellent mask should not irritate the skin, cause headaches or lead to an excessively high concentration of CO2 between the mask and the face. Users tend to remove uncomfortable masks and wear them for less time than is necessary. Breathability is not just crucial for comfort. If breathability is low, air will leak around the edges.
WHO should not dismiss the prevalence of the airborne route
To minimize SARS-CoV-2 transmission through aerosols, WHO highlights the equivalence of surgical masks and respirators in several studies, assuming that HCWs use their masks appropriately. We show that a correct mask fit is difficult to achieve and could be at the origin of this controversy.
Other confounding factors may contribute to the debate, such as i) compliance and quality of the surgical mask or respirator used in the studies; ii) the impact of contamination from improper doffing of masks or ocular inoculation; iii) the effectiveness of ventilation and negative pressure rooms in hospitals, or other infection control measures. These confounders may also have reduced the differences in the effectiveness of surgical masks and respirators in some studies.
Thus, WHO should review its health guidance and practical advice to highlight the relevance of aerosols in the transmission of SARS-CoV-2.
