Epidemiologist: COVID-19 cases rise more in schools when students don't wear masks

U-Mich epidemiologists encouraging the Swiss cheese model of pandemic defense

Photo of Angela Mulka
Group of students wearing protective face masks while raising their hands in class.

Group of students wearing protective face masks while raising their hands in class.

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As students return to schools after months of lockdowns, many people are worried about the safety of their children and transmission of the virus overall.

The role that schools play in transmission of the virus that causes COVID-19 has been difficult to work out, but a University of Michigan epidemiologist is helping to answer that question. Schools do amplify community transmission, but the good news is that some relatively simple mitigation measures can make schools much safer places.

Andrew Brouwer, a mathematical epidemiologist who earned his Ph.D in applied and interdisciplinary mathematics at the University of Michigan, says "K-12 is the largest source of outbreaks in Michigan right now."

"We know that masks effectively reduce the probability of COVID transmission," he continued. "It's true that children generally have milder disease. However, about 50% of children who are hospitalized have no underlying conditions. And, we also have to worry about all the teachers and staff and everyone who's not a child in K-12, as well as all the people these children are going home to — parents and siblings, grandparents and other family members."

Brouwer and other U-Mich epidemiologists, including Marisa Eisenberg, are encouraging layers of defense against COVID-19 in schools because one mitigation strategy is not enough to lower transmission rates in Michigan communities.

The CDC is also recommending prevention strategies to be layered in different ways. And, the number and intensity of the layers can increase if community transmission increases. For instance, as community transmission increases, more holes appear in the defenses, meaning more layers of protection may be needed.

"You know, this sort of Swiss cheese model of COVID exists because no one mitigation measure is good enough, right?" Brouwer said. "Because there's always going to be holes we need to have more and more slices of our cheese to fill the holes. And, because delta is that much more transmissible, it's all that much more important that we're wearing masks, which is part of why CDC guidance evolved from vaccinated people did not need to wear masks to now it's recommended that everyone wears masks in public spaces."

Pictured is the Swiss cheese model of defense strategies against of COVID-19. 

Pictured is the Swiss cheese model of defense strategies against of COVID-19. 

Photo provided/Andrew Brouwer

The Swiss cheese model of COVID defense strategies used by the Michigan Department of Health and Human Services is not edible, but it can save lives. It starts with promoting vaccination against COVID-19 for eligible staff and students. Next, it moves to correctly and consistently using well-fitted masks that cover the nose and mouth. Then, physical distancing comes into play. The next step involves promoting screening and testing for illness and lastly the model ends with ensuring healthy environments and effective ventilation in schools.

More evidence to support masks in schools

New evidence from epidemiological and modeling studies indicate that communities that established mask mandates generally had reduced disease transmission. Controlled studies examining various masks generally show that masks help prevent the spread of virus-laden droplets. Taken together, current scientific data supports the use of masks to reduce the transmission of COVID-19 in the community. You can read the summary of literature here.

Face masks are one of the most effective ways to prevent the spread of COVID-19. An investigation into school outbreaks, supported by the CDC, found that schools without an indoor mask requirement were 3.5 times more likely to have an outbreak than schools in which students and staff did have to wear masks.

This is why the CDC recommends universal indoor masking by all children aged 2 and older, as well as teachers and visitors to schools, regardless of whether they’re vaccinated.

Reducing the viral dose: Inoculum effect

Reducing the viral dose you receive also matters. You may ask, "Aren't we all going to get infected anyway?" But, the amount of exposure matters. We can reduce the severity of infections by the inoculum effect, in which vaccination, masking and social distancing all combine to reduce the viral dose, according to Brouwer.

"One other thing that I think is not as well appreciated is that masks not only reduce the probability of transmission, but even if you get COVID, while wearing a mask, you will get less severe disease," Brouwer said.

"We call it inoculum," he continued. "We know that symptoms correlate really strongly with that sort of initial viral load. For a long time, I think we've suspected that, but recently there have been very convincing epidemiological studies where they're able to compare groups of people that were masking, had ventilation and those that didn't, and although the ones with mitigation had less COVID, those that got it also had less severe disease, and so it's really convincing that masking is part of effective mitigation."

For an individual, being exposed to a small viral dose means milder disease than someone exposed to a large viral dose.

For the community, masks, social distancing and vaccinations mean less disease is spread leading to fewer severe cases.

Why do we still need mitigation measures?

The delta variant spreads easily in indoor spaces when people are unmasked and unvaccinated. Because the delta variant is better at evading the immunity that people have, both natural and vaccine derived, there can be more transmission. More transmission means more people infected, more people infected means more opportunity for hospitalization and death. So, we have to combat by better mitigation, Brouwer said.

Here are some more reasons for mitigation measures outlined by U-Mich epidemiologists: 

1. To protect unvaccinated individuals, those under 12 and those immounocompromised. 

2. To avoid overwhelming the health system by staggering (spacing out) infections. It's important to not let an entire population get infected at once.

3. To protect vulnerable populations. Infections may still be severe in high-risk groups; elderly, underlying conditions.

To determine which mask is best for you, visit the CDC's guidelines on masking here.