The highly transmissible delta variant is a more formidable foe than previously believed, largely due to its ability to infect and be spread by people who are fully vaccinated, according to data gathered by the Centers for Disease Control and Prevention.
A confidential document prepared by the agency cites evidence from a recent outbreak in Massachusetts involving at least 145 people who were infected with the delta variant, which was first detected in India. In the Massachusetts outbreak, the viral loads of the 80 people who were vaccinated were essentially the same as the viral loads of the 65 people who were not vaccinated.
The CDC document also cites reports about so-called breakthrough cases in India. The viral loads of vaccinated people who nonetheless became infected with delta were higher than the viral loads of vaccinated people who were infected with other coronavirus strains, those reports found.
Some of the data in the document was described Tuesday by CDC Director Dr. Rochelle Walensky when she announced new guidance regarding the use of face masks. In parts of the country where coronavirus transmission rates are “substantial” or “high,” everyone — regardless of vaccination status — should wear face coverings when they’re in indoor public spaces, the agency now says.
The confidential document was obtained by the Washington Post and posted on its website.
The delta variant was already known to be about 50% more transmissible than the Alpha variant, which was first detected in the United Kingdom and is itself about 56% more transmissible than the original coronavirus that sparked the global pandemic.
Scientists have also established that people infected with Delta have about 1,000 times more viral particles in their upper respiratory systems than people who are infected with earlier coronavirus strains.
That difference allows delta to jump from person to person just four days after an initial infection, said Chunhuei Chi, director of the Center for Global Health at Oregon State University. With previous strains, it took about six days for that to occur, he said.
This rapid spread helps explain why the delta variant, which is thought to have arrived in the U.S. in March, now accounts for an estimated 82% of recent coronavirus infections in the United States, according to CDC estimates.
The CDC document suggests that breakthrough cases are expected to rise — not only because of delta's enhanced transmission powers, but because the number of vaccinated people is rising as well.