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Federal health officials advised Tuesday that even people who’ve had their Covid-19 shots wear masks in public indoor settings in areas with widespread transmission of the coronavirus, a major setback in the progress in the U.S. epidemic that reflects a surging variant and the country’s ongoing struggles to increase vaccination rates.

The reversal to the Centers for Disease Control and Prevention’s earlier guidance comes as the Delta variant — the most transmissible version of the pathogen yet, by far — is igniting outbreaks and driving hospitalizations up in states with low vaccination rates, like Missouri, Arkansas, and Florida. The variant is responsible for seemingly increasing numbers of breakthrough infections.

The CDC also recommended that schools reopen in the fall with universal indoor masking for all students, teachers, and staff, even among people who are vaccinated — a policy decision that will ultimately be left up to state and local officials.

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“Children should return to full-time, in-person learning in the fall with proper prevention strategies in place,” CDC Director Rochelle Walensky said during a press briefing.

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In a statement, President Biden reiterated Walensky’s call that precautions would enable safer resumption of in-person schooling. “Masking students is inconvenient, I know, but will allow them to learn and be with their classmates with the best available protection,” he said.

Just two months ago, the CDC said that fully vaccinated people no longer needed to wear masks indoors, citing evidence that the immunizations prevented many cases entirely and left those who still got infected “less likely to … transmit [the virus] to others.”

But in addition to spreading more efficiently, Delta seems to have at least some ability to evade the immune response people generate after being vaccinated. Multiple studies have indicated that immunized people are still broadly protected against severe Covid-19 and death, but there is emerging evidence that Delta can cause breakthrough cases at higher rates than other virus versions — and that in some of those infections, people are still harboring large amounts of virus that could be passed to others.

Walensky said that new research indicates that vaccinated people are only contributing a small amount to overall transmission, the vast majority of which is occurring among people who haven’t received immunizations. But the data “unfortunately warrants” the updated recommendations.

“In those cases, those rare cases that we have breakthrough infections, we felt it important for people to understand that they have the potential to transmit virus to others,” she said.

Walensky called masks “a temporary measure” and urged people to get vaccinated, framing the shots as the tools that can suppress transmission rates in lasting ways.

“We really need to work to get these areas in the country that have substantial and high amount of transmission right now down to lower amounts of transmission,” she said.

The masking recommendations for vaccinated people applies to communities with “substantial” or “high” transmission rates, which together the CDC defines as 50 cases or more per 100,000 people over the past seven days. Transmission rates in nearly two-thirds of U.S. counties are in those categories, with wide swaths of Nevada, Utah, and many southern states experiencing those levels of spread, CDC data show. Some communities are reporting several hundred new cases per 100,000 people in the past week — what Walensky called “really an extraordinary amount of viral transmission.” 

Delta’s general transmissibility stems at least in part from changes in its genome that allow it to build up much higher levels of virus in the upper airway, leading to people emitting more virus for longer periods of time, meaning they are more likely to infect others.

An administration official, speaking on the condition of anonymity, told STAT that health experts do not have studies proving that fully vaccinated people are transmitting the virus. Rather, the official said, the updated guidance is based on studies showing that vaccinated people who contract the Delta variant have similarly high levels of virus in their airways, which suggested that they may be infectious to others. With other variants, vaccinated people had substantially lower levels of virus in their noses and throats compared to unvaccinated people.

The new masking guidelines provide an unwelcome turn in the pandemic. As vaccines started rolling out, it was clear they weren’t just protecting people from getting so sick they needed hospital care or they would die, but that they were also drastically cutting transmission. Experts say vaccines are still limiting transmission even with Delta, but not to the same level as before.

It’s taken some time for scientists to tease apart all that Delta can do in vaccinated people. Most concerningly, it has been ripping through unvaccinated populations, leading to another surge in hospitalizations in some states that are almost entirely among people who haven’t been immunized. Experts are still trying to determine if Delta causes more severe disease on average than other variants.

Recent infection clusters that included vaccinated people — and that broadly caused mild and asymptomatic cases — raised worries that Delta was potentially spreading among vaccinated people to an extent. Walensky said the CDC was conducting outbreak investigations of the clusters to determine whether and to what extent vaccinated people were passing the virus on to others.

If vaccinated people are transmitting the virus, masks could help in cases when people are asymptomatic or in the period when they are infectious but haven’t yet developed symptoms. Under existing CDC guidelines, vaccinated people who were showing symptoms were still urged to isolate.

Vaccinated people passing the virus to other vaccinated people isn’t ideal, but given how protective the shots are against the worst outcomes, the bigger concern right now is that vaccinated people could be contributing to the spread of the virus to unvaccinated people. Vaccinated people could also be transmitting the virus to individuals who didn’t mount robust responses to the shots because their immune systems are suppressed, including some with cancer or who have received organ transplants. Children under 12 are also not yet eligible for Covid-19 vaccines.

It’s unclear how the new CDC guidelines will land with an American public that seems eager to move on from the pandemic, even if the pandemic is not done with them. Many people who are vaccinated feel like they’ve done their part and might be unwilling to take a step back. And while health officials hope they can persuade some of the remaining unimmunized people to roll up their sleeves by improving access or addressing concerns, some portion of people have cemented their resistance to the shots.

Under the previous guidelines, unvaccinated people were still supposed to wear masks in public indoor settings.

The CDC says breakthrough cases still appear to be rare given how many people have been vaccinated, and the vast majority of infections are asymptomatic or mild. It’s also thought that because so many of those cases are so tame, many go undetected. However, many experts argue that breakthrough infections that cause no symptoms shouldn’t really be considered as cases. Mild or asymptomatic breakthrough infections are signs that the vaccines — whose top aim is to stave off death and severe illness — are doing their job, experts stress.

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