Passive immunization by low-level exposure to SARS-CoV-2: how masks can do more than protect against infection

Some researchers speculate that universal masking can reduce the dose of SARS-CoV-2 for the mask-wearer, leading to more mild and asymptomatic infection manifestations. Although asymptomatic infections may be harmful for the spread rate of the virus, it could be beneficial if they lead to higher rates of exposure. The following questions arises: could society being exposed to SARS CoV-2 without the unacceptable consequences of severe illness through public masking? Could public masking lead to a greater community-level immunity to slower spread the virus as we wait for a vaccine? [1].

Universal public masking during the pandemic could be one of the most important pillars for disease control. Masking reduce the viral inoculum to which the mask-wearer is exposed, leading to higher rates of mild or asymptomatic infection with SARS-CoV-2. The World Health Organization started recommend population-level face masking on June 5 2020, when the extent of transmission from pre symptomatic or even asymptomatic individuals was clear [2, 3]. 

Face masking is one of the most efficacious interventions to reduce further spread of SARS CoV 2, allowing for less-stringent lock-down requirements in countries adopting this strategy [4]. Countries worldwide have had a range of responses to recommendation on universal masking, and those accustomed to universal population-level masking since the SARS epidemic in 2003 adopted the intervention more readily. 

How to safely wear a face mask. Pan American Health Organization (PAHO) / World Health Organization (WHO). 

There are two main reasons that supports the effectiveness of facial masking:

- To prevent the spread of viral particles from asymptomatic individuals to others [5].

- It is suggested that reducing the inoculum of virus to which a mask-wearer is exposed will result in a milder disease [6, 7].

Face masks filter out a majority of viral particles, but not all, a fact that, combined with the theory that exposure to a lower inoculum or viral dose can make subsequent illness far less likely to be severe supports the role of universal masking as a potent immunization system.

Studies to experimentally examine the dose of virus associated with different levels of disease severity in humans have been limited to non-lethal viruses. It has been described that human volunteers exposed to different doses of Influenza A virus developed more severe symptoms as the doses of administered virus increased [8]. In animal models, hamsters are less likely to contract SARS-CoV-2 infection with a surgical mask partition, as those that did contract the virus with simulated masking had milder manifestations or infection [9]. 

Before facial masking was widely practiced, the proportion of asymptomatic infection with SARS CoV 2 was 15% [10], whereas a more recent review estimated the rate of asymptomatic infections at 40-45% [11]. Closed natural experimental models such as cruise ships can be very illustrative for these studies. While on the Diamond Princess cruise ship the rate of asymptomatic infections was in the 20% range, in a more recent report from a different cruise ship outbreak in which all passengers and staff were provided with surgical masks after an outbreak, 81% of infected patients remained asymptomatic, the majority of them. These studies reinforce the role of masks in mitigating the virulence of SARS-CoV-2 by increasing the immune response without the adverse effects of the disease.

Both countries accustomed to masking since the 2003 SARS-CoV-1 pandemic, as well as those who newly embraced masking early in the COVID-19 pandemic have fared well in terms of rates of severe illness and death. Even when cases have resurged in these areas with population based masking upon re opening, the case fatality rate has remained low, supporting the viral inoculum theory. Asymptomatic infection, although problematic in terms of increasing spread, can be also beneficial as higher rates of asymptomatic infection could lead to higher rates of exposure and therefore higher immunization rates. Exposing society to SARS-CoV-2 without the unacceptable consequences of severe illness could lead to greater community level immunity, slowing down the virus spread. However, it has not been determined yet the effectiveness of antibody and T cell immune responses to different manifestations of the COVID-19. Therefore, the inoculum theory argues that public masking could have a protective effect both for the individual as well as for others, and will allow for the preservation of life along with other control measures such as social distancing, as society gets back to normal. 

How to get your workspace ready to avoid SARS-CoV-2 spreading. World Health Organization. 

Because more studies are needed to confirm the inoculum hypothesis, we should keep following the World Health Organization suggestions to stop the spreading of the virus. People should avoid the ‘3 Cs’: spaces that are Closed, Crowded or involve Close contact; people should meet outside, as it is safer than indoor meetings; and, if people can´t avoid crowded or indoor settings, they must open the windows to increase the amount of natural ventilation and wear a face mask to prevent the spread of aerosols. 

As important as these social measures, it is fundamental to have a good hygiene. People should wash their hands regularly and thoroughly with soap and water or an alcohol-based hand rub; they should avoid touching their eyes, nose and mouth, cover their mouth and nose with the bent elbow or a tissue when coughing or sneezing; and clean and disinfect surfaces frequently, specially those which are regularly touched. 

Together we can stop the spreading of SARS-CoV-2, while scientist keep working for and efficient vaccine or treatment against the virus and its disease.    

References

1. Gandhi, M., C. Beyrer, and E. Goosby, Masks Do More Than Protect Others During COVID-19: Reducing the Inoculum of SARS-CoV-2 to Protect the Wearer. J Gen Intern Med, 2020.

2. He, X., et al., Temporal dynamics in viral shedding and transmissibility of COVID-19. Nat Med, 2020. 26(5): p. 672-675.

3. Gandhi, M., D.S. Yokoe, and D.V. Havlir, Asymptomatic Transmission, the Achilles' Heel of Current Strategies to Control Covid-19. N Engl J Med, 2020. 382(22): p. 2158-2160.

4. Stutt, R., et al., A modelling framework to assess the likely effectiveness of facemasks in combination with 'lock-down' in managing the COVID-19 pandemic. Proc Math Phys Eng Sci, 2020. 476(2238): p. 20200376.

5. Eikenberry, S.E., et al., To mask or not to mask: Modeling the potential for face mask use by the general public to curtail the COVID-19 pandemic. Infect Dis Model, 2020. 5: p. 293-308.

6. Virlogeux, V., et al., Brief Report: Incubation Period Duration and Severity of Clinical Disease Following Severe Acute Respiratory Syndrome Coronavirus Infection. Epidemiology, 2015. 26(5): p. 666-9.

7. Raoult, D., et al., Coronavirus infections: Epidemiological, clinical and immunological features and hypotheses. Cell Stress, 2020. 4(4): p. 66-75.

8. Memoli, M.J., et al., Validation of the wild-type influenza A human challenge model H1N1pdMIST: an A(H1N1)pdm09 dose-finding investigational new drug study. Clin Infect Dis, 2015. 60(5): p. 693-702.

9. Chan, J.F., et al., Surgical mask partition reduces the risk of non-contact transmission in a golden Syrian hamster model for Coronavirus Disease 2019 (COVID-19). Clin Infect Dis, 2020.

10. Buitrago-Garcia, D.C., et al., Asymptomatic SARS-CoV-2 infections: a living systematic review and meta-analysis. 2020: p. 2020.04.25.20079103.

11. Oran, D.P. and E.J. Topol, Prevalence of Asymptomatic SARS-CoV-2 Infection : A Narrative Review. Ann Intern Med, 2020. 173(5): p. 362-367.

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