The Washington PostDemocracy Dies in Darkness

Opinion Indoor masking doesn’t always make sense when everyone is vaccinated

A pedestrian walks past a sign stating that masks are required on the campus of the University of Miami on Aug. 25, 2020, in Coral Gables, Fla. (Wilfredo Lee/AP)
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Joseph G. Allen is an associate professor and director of the Healthy Buildings program at Harvard University’s T.H. Chan School of Public Health. He co-wrote “Healthy Buildings: How Indoor Spaces Drive Performance and Productivity.”

The Centers for Disease Control and Prevention’s recommendations for mask-wearing in areas of “substantial” or “high” spread of covid-19 have resulted in many businesses and universities requiring everyone to wear masks indoors — even if everyone is vaccinated.

This makes no sense. Not only does it rely on flawed metrics to estimate the impact of community spread, but also it ignores all the evidence that vaccines work to limit the spread of the virus.

Let’s first look at how the CDC is defining areas of high transmission. The agency primarily uses two metrics: seven-day-average case numbers per 100,000 people and the rate of coronavirus tests that come back positive. Both have problems.

The first metric might seem reasonable, until one recognizes that there are vast differences in risk by vaccination status and by age. Vaccinated people are 29 times less likely to be hospitalized than the unvaccinated, and children are hospitalized at low rates, too. Despite this, the CDC’s metric is being uniformly applied across the country. Case numbers were once a reliable indicator of population risk, accurately foreshadowing hospitalizations and deaths, but this is changing as more of the country attains some level of immunity.

The second metric — test positivity — is not actually a measure of community risk at all, but rather a way to tell whether we are doing enough testing. This was a useful metric in the early days of the pandemic as we ramped up testing, but people wrongly began to see it as the percent of the population infected. It’s not. Think about who gets tested: It is people with symptoms and who go to the doctor’s office or hospital. Wealthy people also get tested more than those with less means. Millions of people are also using rapid tests at home, going on with their life if negative but getting a lab test to confirm if its positive. This further skews the percent positivity metric.

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Moreover, both metrics are useless in predicting or understanding transmission within a building, which can be minimized with good controls regardless of what’s happening in the broader community. And having a fully vaccinated population is the best control, because the risk of vaccinated-to-vaccinated transmission has been wildly overstated.

We have to think about this in terms of likelihoods. Being vaccinated reduces the chance of getting any infection by about 80 percent or more. Yes, breakthroughs do happen. But symptomatic breakthroughs should not be a major problem — the days of “toughing out” a cold and heading into the office or campus while sick are over. Weed out the symptomatic breakthroughs, and the pool of potentially infectious people becomes even smaller.

What about someone with a breakthrough infection who is asymptomatic? The vaccines help here, too. Several studies have found that the amount of viable virus present in breakthrough infections — meaning the viruses are not just detectable but also can be cultured in a lab — can be lower in people who are vaccinated. The window of infectivity is also shorter for vaccinated people. That’s why another recent study showed that the vaccinated are three times less likely to transmit.

Still, you might say, it can happen. That’s correct, but again, we’re dealing with likelihoods, not absolutes.

Let’s say someone with a breakthrough infection does happen go to work in this shorter period of infectivity and, despite lower viral loads, is emitting virus. The likelihood of transmission will still be very low if everyone is vaccinated, given the vaccine’s protection against infection. And we can lower it even further by improving ventilation and filtration, as I’ve been advocating for more than 20 months.

We overestimate the risk of transmission because we forget about joint probabilities. A lot of things have to line up for someone to be exposed and get covid-19 in a fully vaccinated environment.

On top of this, what’s the risk that a vaccinated individual ends up with severe illness? Here, the vaccines have an impeccable record — proving 95 percent effective at preventing hospitalization and death.

Those who are still worried — either because they are older or immunocompromised or because they are afraid they might bring the virus home to unvaccinated kids — can take steps to reduce their personal risk even further. First, if they are in the higher-risk category, they should get a booster shot, as the CDC just recommended. And if they’re not high-risk but are concerned for other reasons, the CDC opened the door for getting a booster to just about anyone who wants it. This will reduce the likelihood of infection even further.

Second, anyone who wants to wear a mask can still do so. A high-efficiency mask, such as an N95 mask, can reduce exposure by another 95 percent.

Big picture: We’re misunderstanding population risk by relying on faulty metrics of community spread, and then compounding the mistake by using those metrics as the basis for requiring masks indoors when everyone is fully vaccinated. The message we’re sending is that the vaccines don’t work. That’s a dangerous message when the main goal should be to persuade people to get the shots.