(Reuters) – A new study by the U.S. Centers for Disease Control and Prevention showed that three-quarters of individuals who became infected with COVID-19 at public events in a Massachusetts county had been fully vaccinated.

The study, published on Friday, showed that three-quarters of those infected were fully vaccinated, suggesting the Delta variant of the virus is highly contagious.

A separate CDC internal document, first reported by the Washington Post on Friday, described the Delta variant as being as transmissible as chickenpox and cautioned it could cause severe disease.

The new study’s authors recommended that local health authorities consider requiring masks in indoor public settings regardless of vaccination status or the number of coronavirus cases in the community.

The study identified 469 people with COVID-19, 74% of whom were fully vaccinated, following large public events in the state’s Barnstable County. Testing identified the Delta variant in 90% of virus specimens from 133 people.

The viral load was similar in people who were fully vaccinated and those who were unvaccinated, the CDC said.

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, it said.

The finding of the report “is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation,” CDC director Rochelle Walensky said in a statement.

On Tuesday, the CDC reversed course on guidance for mask wearing, calling for their use in areas where cases are surging as a precaution against the possible transmission of the virus by fully vaccinated people.

“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 in a statement.

(Reporting by Manas Mishra in Bengaluru; Editing by Howard Goller)