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Vaccinated People Can Spread Delta Variant, Suggests Data Behind The CDC Mask Advice

Maddie Meyer, Getty Images
Getty Images
Pedestrians walk down Commercial Street in Provincetown, Mass., in May 2020. A study of a new outbreak in Provincetown found that three-quarters of those infected were fully vaccinated.

When the Centers for Disease Control and Prevention revised its mask guidance this week to urge even vaccinated people to wear masks indoors, it was criticized for not citing data in making that move.

Now it has — and the data is sobering.

The study details an outbreak that started July 3 in Provincetown, Mass., involving 469 cases. It found that three-quarters of cases occurred in fully vaccinated people. Massachusetts has a high rate of vaccination: about 69% among eligible adults in the state at the time of the study.

It also found no significant different in the viral load present in the breakthrough infections occurring in fully vaccinated people and the other cases, suggesting the viral load of vaccinated and unvaccinated persons infected with the coronavirus is similar.

The good news is that the vaccines continue to be highly effective against the virus in preventing hospitalization and death. For instance, three studies from Canada, Singapore and Scotland have found that the Pfizer vaccine provides more than 90% protection against hospitalization and death.

The CDC says the finding that fully vaccinated people could spread the virus was behind its move to change its mask guidance.

"High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus," said CDC Director Dr. Rochelle Walensky in a statement on Friday.

"This finding is concerning and was a pivotal discovery leading to CDC's updated mask recommendation. The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones," Walensky said.

At least 882 cases have now been linked to the Provincetown cluster.

As the delta variant continues to spread rapidly through the U.S., the CDC updated mask guidance now recommends people wear masks when inside public spaces in areas where the spread of the virus is "substantial" or "high," regardless of one's vaccination status.
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