CDC calls for masks in schools, hard-hit areas, even if vaccinated


The Centers for Disease Control and Prevention (CDC) once again is recommending that some Americans wear masks indoors. The agency has called for masks in K-12 school settings and in areas of the United States experiencing high or substantial SARS-CoV-2 transmission, even for the fully vaccinated.

Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention

Dr. Rochelle Walensky

The move reverses a controversial announcement the agency made in May 2021 that fully vaccinated Americans could skip wearing a mask in most settings.

Unlike the increasing vaccination rates and decreasing case numbers reported in May, however, some regions of the United States are now reporting large jumps in COVID-19 case numbers. And the Delta variant as well as new evidence of transmission from breakthrough cases are largely driving these changes.

“Today we have new science related to the [D]elta variant that requires us to update the guidance on what you can do when you are fully vaccinated,” CDC Director Rochelle Walensky, MD, MPH, said during a media briefing July 27.

New evidence has emerged on breakthrough-case transmission risk, for example. “Information on the [D]elta variant from several states and other countries indicates that in rare cases, some people infected with the [D]elta variant after vaccination may be contagious and spread virus to others,” Dr. Walensky said, adding that the viral loads appear to be about the same in vaccinated and unvaccinated individuals.

“This new science is worrisome,” she said.

Even though unvaccinated people represent the vast majority of cases of transmission, Dr. Walensky said, “we thought it was important for [vaccinated] people to understand they have the potential to transmit the virus to others.”

As a result, in addition to continuing to strongly encourage everyone to get vaccinated, the CDC recommends that fully vaccinated people wear masks in public indoor settings to help prevent the spread of the Delta variant in areas with substantial or high transmission, Dr. Walensky said. “This includes schools.”

Masks in schools

The CDC is now recommending universal indoor masking for all teachers, staff, students, and visitors to K-12 schools, regardless of vaccination status. Their goal is to optimize safety and allow children to return to full-time in-person learning in the fall.

The CDC tracks substantial and high transmission rates through the agency’s COVID Data Tracker site. Substantial transmission means between 50 and 100 cases per 100,000 people reported over 7 days and high means more than 100 cases per 100,000 people.

The B.1.617.2, or Delta, variant is believed to be responsible for COVID-19 cases increasing more than 300% nationally from June 19 to July 23, 2021.

“A prudent move”

“I think it’s a prudent move. Given the dominance of the [D]elta variant and the caseloads that we are seeing rising in many locations across the United States, including in my backyard here in San Francisco,” Joe DeRisi, PhD, copresident of the Chan Zuckerberg Biohub and professor of biochemistry and biophysics at the University of California San Francisco, said in an interview.

Dr. DeRisi said he was not surprised that vaccinated people with breakthrough infections could be capable of transmitting the virus. He added that clinical testing done by the Biohub and UCSF produced a lot of data on viral load levels, “and they cover an enormous range.”

What was unexpected to him was the rapid rise of the dominant variant. “The rise of the [D]elta strain is astonishing. It’s happened so fast,” he said.


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