How Masks Reduce the Severity of COVID-19 Infections?

August 3, 2020
Takeaway: As we know masks decrease the spread of infection. And in those that become infected while wearing a mask, symptoms are less severe. So while masks have been recommended to protect others, it appears that there is also a direct benefit to the mask wearer.


This is a summary from our recent review of a Grand Rounds presentation provided by UCSF titled: “How the virus gets in, and how to block it: Aerosols, Droplets, Masks, Face Shields and more.” 

For simplicity, I will use these two names interchangeably for the duration of this summary.  This, of course, does not constitute medical advice nor does it create a doctor-patient relationship between you (the reader) and me.

Monica Gandhi, UCSF Professor of Medicine; Associate Chief of Division of HIV, Infectious Diseases, and Global Medicine at ZSFG; Director of the UCSF Center for AIDS Research; and Medical Director of the HIV Clinic, Ward 86, ZSFG presented a very strong argument for masks as a means for protecting both the community and the individual wearing the mask.

By now, most people have heard about the cardinal symptoms of COVID-19, cough, fever, shortness of breath, loss of sense of smell or taste.   One unique feature of SARS-CoV-2 is asymptomatic infections --  that is --  people infected with COVID-19 but have no symptoms of infection.  The rate of asymptomatic infection is likely around 40%.

A study covering the Mission district of San Francisco did extensive COVID-19 testing and followed all people who tested positive. That study found that 42% of the positive individuals never went on to develop symptoms of COVID. A similar CDC study mirrors this result.  

Asymptomatic infections are truly a double edge sword.  The benefit of an asymptomatic infection is that many go on to develop immunity to COVID-19.  A lot of hopeful papers are coming out about long-term cellular immunity with coronaviruses. 

The bad news of asymptomatic infection is that a person may not be aware that they have an infection and can unwittingly spread the illness to others.  Data from February and March shows high viral loads shed from your nose and mouth even without symptoms.  Because of this, the CDC recommends routine mask use to protect our communities.  

As a business owner, asymptomatic infections are clearly problematic.  An employee, with no illness symptoms, may avoid detection from a good employee health screening program, report to work and unknowingly spread the infection to other workers.  

Viral “dose” as it is related to severity of disease: 

Evidence that reduced viral dose leads to less severe disease is an age old concept first published 1938, study in mice. The more virus researchers gave the mice, the sicker they got. A recent study simulated masking in hamsters with SARS-CoV-2 and showed “masked” hamsters were less likely to get disease and if they got disease it was more mild.  

This is because the masked animal likely received a smaller dose of the virus as a benefit of wearing a mask.  Multiple studies show that masking helps to decrease transmission of infection.  N95 masks filter out 90-95% of viral particles, and surgical and cloth masks filter 65-85%. 

In humans, we have no studies purposely exposing humans to COVID-19, but we do see that populations with higher exposure to COVID-19 have more severe illness.  This is seen in areas of high viral density such as in healthcare workers and in household contacts (both of which would be expected to be exposed to a higher doses of virus than other methods of exposure).  With higher concentrations of virus at initial exposure, we see higher death rates.  

If this is true, do we see less severe and/or more asymptomatic infections under “masked” circumstances? 

There is some compelling data that suggests yes.  

First, let’s compare two cruise ship outbreaks.  For the Diamond Princess cruise ship outbreak, many people developed infection and 18% of the infections were asymptomatic.  Guests and crew from another cruise ship from Argentina with an outbreak were given masks. The rate of infection was still fairly high: 128 of the 217 passengers.  But 81% of the infections were asymptomatic (CDC July 14, 2020).  That is a remarkable difference. 

Similar findings have been seen in other settings when outbreaks occur despite universal mask use.  

In an outbreak in a pediatric dialysis unit where all individuals were masked, all cases were asymptomatic.  In two outbreaks in food plants where workers were masked,  of those who became infected, 95% had no symptoms. 

Masks allows us to to re-open our economy

As a business owner, I don’t want any outbreak in my place of business, but if an outbreak occurs, I would certainly hope my employees were less likely to become severely ill.  

The Czech Republic mandated facial masking on March 23rd.  The country subsequently removed the mask order and while cases went up with the opening; the death rate was flat (again, less severe infections). (WHO situation report 7/10/20)

Another study provided a model that compared social distancing, shelter-in-place, and masks use.  When at least 80% of the population used masks, the simulation showed that strategy as providing the lowest death rates (lower even than continued shelter-in-place). Of course masking requires individual behavior change, but there is strong evidence it can reduce the severity of the disease. 

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