NYTimes: C.D.C. Reports Vaccinated People May Spread the Virus, Though Rarely

In yet another unexpected and unwelcome twist in the pandemic, the Centers for Disease Control and Prevention released on Friday a report strongly suggesting that fully immunized people with so-called breakthrough infections of the Delta variantcan spread the virus to others just as readily as unvaccinated people. 

The vaccines remain powerfully effective against severe illness and death, and the agency said infections in vaccinated people were comparatively rare. But the revelation follows a series of other recent findings about the Delta variant that have upended scientists’ understanding of the coronavirus.

In the new report, which was intended to explain the agency’s sudden revision to its masking advice for vaccinated Americans, the C.D.C. described an outbreak in Provincetown, Mass., this month that quickly mushroomed to 470 cases statewide as of Thursday. 

Three-quarters of the infected were fully immunized, and the Delta variant was found in most of the samples that were genetically analyzed. Vaccinated and unvaccinated people who were infected carried high levels of the virus, the agency reported. 

“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,” Dr. Rochelle Walensky, director of the C.D.C., said on Friday. 

“This finding is concerning and was a pivotal discovery leading to C.D.C.’s updated mask recommendation,” she added. 

An internal agency document, which was obtained on Thursday night by The New York Times, suggested even greater alarm among C.D.C. scientists and raised harrowing questions about the virus and its trajectory. 

The Delta variant is about as contagious as chickenpox, the document noted, and may contribute to 35,000 symptomatic infections per week among vaccinated Americans. “Given higher transmissibility and current vaccine coverage, universal masking is essential,” the document said.

Still, the incidence of infection in unvaccinated people is eight times greater, meaning that breakthrough infections overall are infrequent, according to the agency. On Friday, the Kaiser Family Foundation reported that the rate of breakthrough cases is less than 1 percent among fully vaccinated people in states that keep such data.