A new study of the US Centers for Disease Control and Prevention (CDC) shows the Delta variant produced similar amounts of virus in vaccinated and unvaccinated people if they get infected.
The study, published by the CDC on Friday, focused on 469 Covid-19 cases identified among Massachusetts residents who had travelled to Barnstable County, a summer vacation destination, during July 3 to 17, the Xinhua news agency reported.
A total of 346 cases, about 74 per cent, occurred in fully vaccinated people, according to the study.
Testing identified the Delta variant in 90 per cent of specimens from 133 patients.
Cycle threshold values were similar among specimens from patients who were fully vaccinated and those who were not, according to the study.
The study demonstrated that Delta infection resulted in similarly high SARS-CoV-2 viral loads in vaccinated and unvaccinated people, said CDC Director Rochelle Walensky.
"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," she said.
Walensky said that this finding is concerning and was a pivotal discovery leading to CDC's updated mask recommendation.
The CDC updated its masking recommendation on Tuesday, urging vaccinated Americans to resume wearing masks in schools and in public indoor spaces in Covid-19 hot spots across the country.
"The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones," she said.
The CDC suggested jurisdictions to consider expanded prevention strategies, including universal masking in indoor public settings, particularly for large public gatherings that include travellers from many areas with differing levels of SARS-CoV-2 transmission.