Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, adjusts her face mask during a Senate Health, Education, Labor and Pensions Committee hearing on the federal coronavirus response on Capitol Hill in Washington, in this Thursday, March 18, 2021, file photo. Credit: Susan Walsh / AP

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Max Nisen is a Bloomberg Opinion columnist covering biotech, pharma and health care.

Just two months after the U.S. Centers for Disease Control and Prevention said it was generally safe for those vaccinated against COVID-19 to drop their masks, the agency switched course and now recommends that even the inoculated wear face coverings indoors in areas where the highly transmissible delta variant is spreading rapidly.

The guidance change may provoke some whiplash — and it’s already generated some backlash, if social media is any measure — but it makes sense. Here’s why.

The biggest reason to encourage wearing masks is to protect others, many of whom are still unvaccinated. The CDC justified its initial rollback with data that suggested vaccinated people were less likely to transmit the virus. But that was before the delta variant emerged. Delta spreads faster because it evades protective antibodies and grows more copies of itself, likely contributing to lower protection against symptomatic infection by even highly effective mRNA vaccines. There isn’t definitive data to confirm it yet, but if vaccines are less effective at wiping out delta and there’s more of it, it’s only logical to expect more transmission. The CDC’s decision was based in part on data suggesting that vaccinated people infected with delta who transmit to others had higher viral loads, according to a Tuesday news conference led by Director Rochelle Walensky. Viral load is key to transmissibility, and it was lower in vaccinated people who contracted other strains. The CDC findings dovetail with a preprint study of breakthrough infections in India, which found that vaccinated people infected with the delta variant had higher respiratory viral loads and were more likely to spread Covid than those infected with other strains. Fortunately, the variant is not any better at getting around physical barriers such as masks, which are especially effective at stopping transmission.

The risk of a COVID-19 infection rises in the face of delta and growing case counts, even for people with two shots. While the chances of a bad outcome for any vaccinated individual are low, they aren’t zero.  Israel reports about 90 percent efficacy against severe disease and hospitalization for the mRNA vaccine made by Pfizer Inc. and BioNTech SE. That’s lower than the rate seen in clinical trials, which you’d expect in the real world. But it does mean that if there are thousands of cases of COVID-19 among vaccinated individuals, we will see more deaths and hospitalizations among this group, even if the proportion is drastically lower than in the unvaccinated. Wearing masks doesn’t just reduce the risk of infection — it also limits the amount of virus encountered when it doesn’t provide complete protection, leading to better disease outcomes.

The delta outbreak in India earlier this year was exceptionally deadly, highlighting the risk of the variant. But it was also comparatively short; the country’s surge began in March and it passed its infection peak in mid-May. The same dynamic may be occurring in the U.K., which is seeing cases drop, though its recent reopening means it’s worth waiting a few weeks to see if the downward trend holds. While vaccinated people will account for a smaller proportion of both transmission and deaths there and in the U.S. as delta takes hold, margins matter, and masking in the highest-risk situations is a relatively easy way to help ensure the U.S. outbreak is shorter and less damaging. That will help masks go away more permanently sooner.

I can understand the frustration with this shift. It feels like mixed messaging from the CDC, which hasn’t always been a model of consistency. But it’s far better to have public health leaders willing to change their minds in response to changing evidence and stay ahead of the data instead of waiting too late.