New science has again prompted the Centers for Disease Control and Prevention to update its mask guidelines.

The CDC recommended on Tuesday that fully vaccinated people wear masks in public indoor settings "in areas with substantial and high transmission." That includes a large swath of the US right now, including the vast majority of counties in the South.

The CDC recommended that all teachers, staff, students, and visitors at K-12 schools mask up as well.

The goal of these new guidelines, CDC director Rochelle Walensky said, is to "help prevent the spread of the Delta variant and protect others."

The CDC said in May that vaccinated people don't need masks, a recommendation based partly on data indicating that vaccinated people were less likely to transmit the virus to others. But the Delta variant – now the dominant strain in the US – behaves differently than previous versions of the virus, Walensky said.

"Information on the Delta variant from several states and other countries indicates that, on rare occasions, some vaccinated people infected with the Delta variant after vaccination may be contagious and spread the virus to others," Walensky said during a press call on Tuesday.

"This new science is worrisome and unfortunately warrants an update to our recommendations."

Walensky said CDC investigations have found that the amount of virus present in vaccinated people infected with Delta is similar to the levels found in unvaccinated people with Delta infections.

That's an indication that vaccinated people can easily transmit the virus — even if they're less likely to get sick on the whole.

Still, Walensky added, "the vast majority of transmission, the vast majority of severe disease, hospitalization, and death is almost exclusively happening among unvaccinated people".

The CDC estimated last week that unvaccinated people represent about 97 percent of hospitalized COVID-19 cases in the US. Vaccines reduce the risk of a symptomatic Delta infection sevenfold, Walensky said. And the risk of hospitalization and death from Delta drops twentyfold after someone has been vaccinated, she added.

But in areas of high transmission, Walensky said, about one in 20 – or even one in 10 – of a person's contacts could lead to a breakthrough infection (a case diagnosed after someone is fully vaccinated). That's assuming vaccines are 90 to 95 percent effective.

Vaccines still work well against Delta

So far, vaccines appear to be slightly only less effective against Delta than against other strains.

A recent study in the New England Journal of Medicine found that Pfizer's two-dose vaccine reduces the risk of a symptomatic Delta infection by 88 percent – compared to 95 percent against the original strain. But just one shot of the vaccine reduced that risk by only 36 percent.

Meanwhile, a Canadian study that's still awaiting peer review found that a single dose of Pfizer's shot was 56 percent effective at preventing symptomatic infections caused by Delta after two weeks. That rate was 72 percent for Moderna's shot.

Breakthrough infections could be more common with Johnson & Johnson's shot than Pfizer's or Moderna's, since the efficacy of that vaccine is lower: J&J's shot was found to cut the risk of moderate and severe COVID-19 by 66 percent globally in clinical trials.

Yet South African researchers recently found that 94 percent of breakthrough infections among J&J recipients were mild, including those infections caused by Delta.

Delta is more transmissible than previous strains, though – meaning that statistically it will lead to more hospitalizations and deaths among vaccinated people, too, since it spreads so easily.

An analysis from Public Health England found that Delta was associated with a 60 percent increased risk of household transmission compared with the Alpha variant discovered in the UK, though more recent estimates suggest the difference is closer to 40 percent.

The Alpha variant is already about 50 percent more transmissible than the original strain, according to the CDC.

This article was originally published by Business Insider.

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