Understanding that New England Journal Masking In Schools Study

Nicely analyzed data suggests lifting mask mandates in schools led to higher infection rates.

F. Perry Wilson, MD MSCE


On March 26th, 2022, Hawaii became the last state in the United States to lift its indoor mask mandate. By the time the current school year started, there were essentially no public school mask mandates either.

Whether you viewed the mask as an emblem of stalwart defiance against a rampaging virus, or a scarlet letter emblematic of the overreaches of public policy, you probably aren’t seeing them much anymore.

And yet, the debate about masks still rages. Who was right, who was wrong? Who trusted science and what does the science even say? If we brought our country into marriage counseling, would we be told it is time to move on? To look forward, not back? To plan for our bright future together?

Perhaps. But this question isn’t really moot just because masks have largely disappeared in the US. Variants may emerge that lead to more infection waves — and other pandemics may occur in the future. And so I think it is important to discuss a study that, with quite rigorous analysis, attempts to answer the following question: Did masking in schools lower students’ and teachers’ risk of COVID?

We are talking about this study, appearing in the New England Journal of Medicine.

The short version goes like this.

Researchers had access to two important sources of data. One — an accounting of all the teachers and students (more than 300,000 of them) in 79 public, non-charter school districts in Eastern Massachusetts who tested positive for COVID every week. Two — the date that each of those school districts lifted their mask mandates or (in the case of two districts) didn’t.

Right away, I’m sure you’re thinking of potential issues. Districts that kept masks even when the statewide ban was lifted are likely quite a bit different from districts that dropped masks right away. You’re right, of course — hold on to that thought we’ll get there.

But first — the big question — would districts that kept their masks on longer do better when it comes to the rate of COVID infection?

It appears that they do…

What you see here are COVID rates in school before and after mask mandates were lifted (those dotted lines).

So — in the beginning— when everyone was masking, case rates were pretty similar (I’ll prove this to you more in a second).

Statewide mandates were lifted in late February — and most school districts removed their mandates within a few weeks — the black line are the two districts (Boston and Chelsea) where mask mandates remained in place.

As time marched on, you can see the case rates in the various districts spread out — with districts that kept masks on longer doing better than those that took them off, and districts that kept masks on the whole time doing best of all.

It’s easier to see in this graph, which normalizes the COVID-cases to the rate in the districts that kept masking, so we don’t have the wave-effects.

Prior to the mask mandate lifting, you see very similar covid rates in districts that would eventually remove the mandate and those that do not, with a bit of noise around the initial Omicron wave which saw just a huge amount of people get infected.

And then, after the mandate was lifted, separation. Districts that held on to masks longer had lower rates of COVID infection.

In all, over the 15-weeks of the study, there were roughly 12,000 extra cases of COVID in the mask-free school districts, which corresponds to about 35% of the total COVID burden during that time. And, yes, kids do well with COVID — on average. But 12,000 extra cases is enough to translate into a significant number of important clinical outcomes — think hospitalizations and post-covid syndromes. And of course, maybe most importantly, missed school days. Positive kids were not allowed in class no matter what district they were in.

Ok — I promised we’d address confounders. This was not a cluster-randomized trial, where some school districts had their mandates removed based on the vicissitudes of a virtual coin flip, as much as many of us would have been interested to see that. The decision to remove masks was up to the various school boards — and they had a lot of pressure on them from many different directions. But all we need to worry about is whether any of those things that pressure a school board to keep masks on would ALSO lead to fewer COVID cases. That’s how confounders work, and how you can get false results in a study like this.

And yes — districts that kept the masks on longer were different than those who took them right off. But check out how they were different.

The districts that kept masks on longer had more low-income students. More Black and Latino students. More students per classroom. These are all risk factors that increase the risk of COVID infection. In other words, the confounding here goes in the opposite direction of the results. If anything, these factors should make you more certain that masking works.

The authors also adjusted for other factors — the community transmission of COVID-19, vaccination rates, school district sizes, and so on. No major change in the results.

One concern I addressed to Dr. Ellie Murray, the biostatistician on the study — could districts that removed masks simply have been testing more to compensate, leading to increased capturing of cases?

She directed me to the supplementary data — seen here.

If anything, the schools that kept masks on were testing more than the schools that took them off — again that would tend to imply that the results are even stronger than what was reported.

Is this a perfect study? Of course not — it’s one study, it’s from one state. And the relatively large effects from keeping masks on for one or two weeks require us to really embrace the concept of exponential growth of infections, but, if COVID has taught us anything, it is that small changes in initial conditions can have pretty big effects.

My daughter, who goes to a public school here in Connecticut, unmasked, was home with COVID this past week. She’s fine. But you know what? She missed a week of school. I worked from home to be with her — though didn’t test positive. And that is a real cost to both of us that I think we need to consider when we consider the value of masks. Yes, they’re annoying — but if they keep kids in school might they be worth it? Perhaps not for now, as cases aren’t surging. But in the future, be it a particularly concerning variant, or a whole new pandemic, we should not discount the simple, cheap, and apparently beneficial act of wearing masks to decrease transmission.

A version of this commentary first appeared on Medscape.com.



F. Perry Wilson, MD MSCE

Medicine, science, statistics. Associate Professor of Medicine and Public Health at Yale. New book “How Medicine Works and When it Doesn’t” available now.