CDC says fully vaccinated people spread the Delta variant and should wear masks: ‘This new science is worrisome’

• The CDC updated its guidelines on Tuesday to recommend masks indoors, even for vaccinated people.
• The Delta variant makes it easier for vaccinated people to transmit the virus, the CDC said.
• Vaccinated and unvaccinated people infected with Delta may have similar viral loads.
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New science has once 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 most US states right now, except for some in the Midwest and on the East Coast.

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

The goal of these new recommendations, 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, in part, 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, in rare occasions, some vaccinated people infected with the Delta variant after vaccination may be contagious and spread the virus to others," Walensky said on a Tuesday press call. "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% of hospitalized COVID-19 cases in the US. According to Walensky, vaccines still reduce the risk of a person developing a symptomatic Delta infection by seven-fold. The risk of hospitalization and death from Delta goes down 20-fold after someone has been vaccinated, she added.

But in areas of high transmission, Walensky said, around 1 in 20 - or even 1 in 10 - of a person's contacts could potentially lead to a breakthrough infection (a COVID-19 case diagnosed after someone is fully vaccinated). That's assuming the current vaccines are around 90% to 95% effective.

Vaccines still work well against Delta

So far, vaccines only appear to be slightly less effective against Delta than against other strains - if administered in full.

A recent study in the New England Journal of Medicine suggests that Pfizer's two-dose vaccine reduces the risk of a symptomatic Delta infection by 88% - down from 95% against the original strain. Just one shot of the vaccine only reduced that risk by 36%, though.

Meanwhile, a Canadian study that's still awaiting peer review found that a single dose of Pfizer's shot was 56% effective at preventing symptomatic infections caused by Delta after two weeks. That rate was 72% 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% globally in clinical trials. However, South African researchers recently found that 94% of breakthrough infections among J&J recipients were mild, including those infections caused by Delta.

But Delta is more transmissible than previous strains - meaning, statistically, it will lead to more hospitalizations and deaths among vaccinated people.

An analysis from Public Health England found that Delta was associated with a 60% 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%. The Alpha variant is already about 50% more transmissible than the original strain, according to the CDC.