Growing evidence shows universal masking in public could dampen the feared “second wave” of COVID-19, says Monica Gandhi, MD, MPH, an infectious disease physician at the University of California, San Francisco.

“I am profoundly hopeful of that when I look at other countries,” she says. “As they open up, the most effective measure to prevent resurgence is universal masking.”

Although the issue may be hopelessly politicized, universal masking is considered the best option to reopen the economy amid a continuing pandemic.

Gandhi cites a study of a coronavirus outbreak on a cruise ship where all passengers were issued surgical masks. Researchers found 81% of those who tested positive for COVID-19 remained asymptomatic.1 In a paper in press, Gandhi and colleagues propose an “inoculum” theory hypothesizing masks lower the viral dose emitted and received in a setting of a universal masking.2 It is well established that a mask acts as source control, protecting others from the wearer.

“But there is a second benefit that has received less attention,” Gandhi and co-authors noted. “Exposure to a lower inoculum or dose of any virus (whether respiratory, gastrointestinal, or sexually transmitted) can make subsequent illness far less likely to be severe. Increasing rates of asymptomatic and mild infection with COVID-19 seen over time, in the setting of masking, supports this theory.”

Lower rates of COVID-19 mortality are documented in Asian countries where wearing a mask in public is a cultural norm, the authors emphasized.

“While there is some correlation between universal masking and number of COVID-19 cases, there is a near-perfect correlation between public masking and suppression of COVID-related death rates,” they concluded. “Case fatality from COVID-19 is universally low in regions with universal masking.”

Changing the Message

There is confused messaging and political divisiveness on this issue, with the initial rationale that wearing a mask is to protect others. Gandhi argues that in a public setting, surgical masks may afford the wearer some protection by lowering the viral inoculum inhaled.

“The beginning message was that it protects others. But in a society that has not exactly been altruistic — especially in the current climate — that has not been the most effective way to convince people to wear masks,” she says. “The message needs to be that masks protect both. It protects others and it protects you, as well. It makes sense that it would protect you. You can’t contract it directly through your skin; you get it into your body through your mouth or nose. Wearing a mask also protects you from touching your mouth and nose.”

Mask-wearing in public is not a cultural norm in the United States, Gandhi concedes, but notes that neither is mass isolation of people in their homes.

“We have figured out a lot about this virus in the last few months,” she says. “Before it was ‘radioactive’ — can you touch a surface and get it? We [know] that it is shed from the nose and mouth. That makes it so simple in a way; all you have to do is cover the nose and mouth.”

There could be some benefit if any resulting infection is mild or asymptomatic, particularly if it is found that such cases develop immunity and thus increase protection of the herd.

“As the economy opens up, universal masking may not prevent exposure but potentially lead to only mild disease,” Gandhi and colleagues stated. “One model found that if 80% of the population wears a moderately effective mask, nearly half of the projected deaths over the next two months could be prevented.3 That means less illness, fewer deaths, and a safer reopening of society.”


  1. Ing AJ, Cocks C, Green JP. COVID-19: In the footsteps of Ernest Shackleton. BMJ Thorax [Epub ahead of print].
  2. Gandhi M, Goosby E, Jha A. Universal masking protects the wearer and your neighbor: The inoculum theory. June 2020. In press.
  3. Kai D. et al. Universal masking is urgent in the COVID-19 pandemic: SEIR and agent based models, empirical validation, policy recommendations, April 22, 2020. In press.