The recent article published in the New England Journal of Medicine sets out the unproven but promising theory that the universal use of masks could help reduce the severity of the virus and ensure that a higher percentage of new infections are asymptomatic.
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The use of the mask could reduce the severity of the infection and allow an increased percentage of new infections to be asymptomatic. According to the authors, if this hypothesis is confirmed, then the universal use of the mask could be considered a method of variolation, which would generate immunity and thus slow down the spread of the virus while waiting for the vaccine.
A growing body of evidence suggests that it is the amount of virus to which someone is exposed at the time of infection - the "infectious dose" or “viral load” - that determines the severity of their disease. Indeed, an extensive study published in The Lancet last month found that "viral load at diagnosis" was an "independent predictor of mortality" in hospital patients. Wearing the masks could therefore reduce the infectious dose to which the wearer is exposed and, consequently, the impact of the disease, as the masks filter out a few droplets containing the virus.
If this theory is confirmed, then, according to the researchers, the use of masks throughout the population could ensure that a higher percentage of Covid-19 infections are asymptomatic. Better still, since data has emerged in recent weeks suggesting that there may be strong immune responses even from infection with mild or asymptomatic symptoms, researchers say that any public health strategy that helps reduce the severity of the virus - such as the use of masks - should also increase immunity at the population level. This is because even a low viral load may be sufficient to induce an immune response, which is what a vaccine typically does.
This hypothesis needs to be supported by more clinical studies, but there are several indications that bode well. Experiments on hamsters have suggested a connection between dose and disease. Earlier this year, a team of researchers in China discovered that hamsters housed behind a barrier made of surgical masks were less likely to be infected with the coronavirus. In addition, those infected with the virus were less seriously ill than other animals without surgical masks to protect them.
Some observations in humans also seem to support this hypothesis. For example, during a coronavirus epidemic on an Argentine cruise ship put into isolation, where passengers were fitted with surgical masks and staff with N95 masks, the asymptomatic infection rate was 81%. This compares with 20% of previous epidemics on cruise ships without the use of protective masks.
Dr. Monica Gandhi, an infectologist at the University of California, San Francisco, and one of the paper authors, stressed that the article has its limitations and should not be interpreted as anything other than a theory. "To test the variolation hypothesis," she said, "we will need further studies comparing the strength and duration of SARS-CoV-2-specific T-cell immunity among people with an asymptomatic infection and those with symptomatic infection, as well as a demonstration of the natural slowdown in the spread of SARS-CoV-2 in areas with a high percentage of asymptomatic infections.
Prof. Julian Tang, an honorary associate professor of Respiratory Sciences at the University of Leicester, shared Dr. Gandhi's cautious optimism. "This idea of variolation - a method of protection, for example, used against smallpox before vaccination - is reasonable and can be added to the protective physical effects of mask use throughout the population.
Low-level stimulation of the immune system, exposed to low levels of SARS-CoV-2, can induce an immune response, but without the obvious disease," she said. He added: "This is after all the response to a typical vaccine, a situation where the immune system is subclinically stimulated to produce protective immune responses to fight the infection in case of future exposure to the antigen. Of course, studies are needed to confirm this effect".
1. Gandhi M, Rutherford GW. Facial Masking for Covid-19 - Potential for "Variolation" as We Await a Vaccine [published online ahead of print, 2020 Sep 8]. N Engl J Med. 2020;10.1056/NEJMp2026913. doi:10.1056/NEJMp2026913
2. Hayes G. Face masks could be giving people Covid-19 immunity, researchers suggest. The Telegraph. 12 september 2020