THE U.S. CDC updated its guidelines Tuesday to recommend masks, even for fully vaccinated people in public indoor settings “in areas with substantial and high transmission.”

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 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 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.”

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 1 in 20 — or even 1 in 10 — of a person’s contacts could lead to a breakthrough infection (a case diagnosed after someone is fully vaccinated).

So far, vaccines appear to be slightly 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.

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. 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.(SD-Agencies)