Most Covid-19 cases in Massachusetts outbreak among vaccinated: CDC

People get vaccinated at the UMass Memorial Health Care Covid-19 Vaccination Centre in Worcester, Massachusetts, on April 22, 2021.
People get vaccinated at the UMass Memorial Health Care Covid-19 Vaccination Centre in Worcester, Massachusetts, on April 22, 2021.PHOTO: AFP

WASHINGTON (REUTERS) - Three-quarters of people infected with Covid-19 at July public events in a town on Cape Cod in Massachusetts were fully vaccinated, a study by the United States Centres for Disease Control and Prevention (CDC) showed.

The study, published on Friday (July 30), suggested the Delta variant of the virus was highly contagious. The outbreak occurred in Provincetown on Cape Cod, according to Barnstable County health authorities.

The CDC study found vaccinated individuals had a similar amount of virus presence as the unvaccinated, suggesting that, unlike with other variants, vaccinated people infected with the Delta variant could transmit the virus, the CDC said.

CDC director Rochelle Walensky said this was a "pivotal discovery" leading to CDC's recommendation this week that masks be worn in areas where cases were surging as a precaution against possible transmission 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," Dr Walensky said.

The CDC said 469 cases were found among Massachusetts residents from July 3 to 26 related to the Cape Code outbreak.

Of those, 74 per cent were among fully vaccinated people. The CDC said its study excluded residents of 22 other states. Barnstable county reported that as of July 30, 934 total cases had been associated with the outbreak.

The CDC said that overall, 79 per cent of the vaccinated individuals who were infected with Covid-19 also reported symptoms such as cough, headache, sore throat and fever. Four had to be hospitalised, the CDC said.

Vaccinated individuals had received one of the three available shots made by Pfizer and BioNTech, Moderna or Johnson & Johnson, the data showed.

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

The CDC said that people with Covid-19 reported having been at densely packed indoor and outdoor events including bars, restaurants, guest houses and rental homes.