"In rare occasions, some vaccinated people infected with the delta variant after vaccination may be contagious and spread the virus to others," she told reporters on Tuesday.
"This new science is worrisome and unfortunately warrants an update to our recommendations."
Previous recommendations from the agency were based on COVID-19 monitoring data that indicated vaccinated people rarely transmit the virus to others.
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.
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). That's assuming vaccines are 90% to 95% effective.
However, about two-thirds of the general population have COVID-19 antibodies and 67.6% of India's population were exposed to the disease, the fourth serosurvey conducted by ICMR during the June-July 2021 period found.