An unproven but exciting new theory suggests that even if face masks don’t fully protect the user, they work to reduce the viral load — and this, in turn, is likely to trigger immunity following an asymptomatic infection, somewhat similar to vaccine inoculation.

From the early stages of the pandemic, infectious experts have supported the use of face masks as a way to reduce COVID-19 transmission. This was especially prevalent in Asian countries, where face masks were much more commonplace to begin with, but eventually caught on elsewhere in the world. The evidence shows a strong link between wearing face masks and reducing community transmission, and face masks have become a pillar of our fight against COVID-19.

But they might also be doing something else. They might be giving some people immunity without a visible symptom.

The theory is unproven but plausible. It was addressed in a commentary published in the New England Journal of Medicine, one of the most prestigious medical journals in the world.

“Universal masking seems to reduce the rate of new infections; we hypothesize that by reducing the viral inoculum, it would also increase the proportion of infected people who remain asymptomatic,” write Monica Gandhi and George Rutherford, two infectious experts.

The idea is that masks make it so that even if you get infected, the initial viral load that infects you is quite small, which reduces the intensity of the symptoms or even eliminates them completely.

This has been speculated before, though it’s still unclear to what extent this initial viral load affects the severy of the disease. However, Gandhi and Rutherford call on several observational studies that back their hypothesis up.

For instance, the typical rate of asymptomatic infection is estimated to be around 40% by the CDC. However, in places with universal or near-universal face masking, the asymptomatic infection rates are much higher, even reaching 80%. Countries that have adopted widespread face masks tend to have much better survival rates, which also suggests a shift to more asymptomatic cases.

More localized studies also seem to back this idea up. For instance, in an outbreak on a closed Argentinian cruise ship where passengers were provided with surgical masks (and staff had N95 masks), the asymptomatic rate was 81% (compared to 20% for other ship outbreaks without universal masking). In US food processing plants where workers wore masks every day, the proportion of asymptomatic cases was even higher, reaching 95%.

The idea is also supported by a study on animal models, where Syrian hamsters administered higher doses of virus exhibited more severe manifestations of the disease.

While this theory is still somewhat speculative, it would fit very well with the observed data. If it is indeed true, it would suggest that in addition to helping limit prevention, face masks would also work as a vaccine of sorts, improving immunity after an asymptomatic course (or at the very least reducing the intensity of the symptoms). Ultimately, to keep the disease in check, we need to reduce both the transmission rate and the severity of the disease — and masks seem to help on both ends.

While experts are still not sure how COVID-19 immunity works and how long it lasts, reinfection rates seem to be rare and in general, the infection seems to grant immunity, at least for a few months. Promising data indicate that even mild or asymptomatic COVID-19 cases grant immunity.

As we wait for a vaccine with winter lurking in the northern hemisphere, we need every bit of help we can get. If there is an added benefit to face masks, that would offer even more support for their widespread usage. Researchers call for more studies to analyze this theory.

“To test our hypothesis that population-wide masking is one of those strategies, we need further studies comparing the rate of asymptomatic infection in areas with and areas without universal masking,” the researchers conclude.