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Syringes with the Pfizer vaccine are prepared for a coronavirus disease (COVID-19) vaccine clinic aimed at youths ages 12 or older at La Colaborativa in Chelsea, Massachusetts, on Jun 11, 2021. (Photo: REUTERS/Brian Snyder) - 

Three quarters of people infected with COVID-19 at public events in a Massachusetts town were fully vaccinated, a study by the US Centers for Disease Control and Prevention (CDC) showed.

The study, published on Friday without naming the town, suggested the Delta variant of the virus was highly contagious.

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," Walensky said.

Overall, 79% 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 hospitalized.

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

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

Multiple events in the town in Barnstable county, Massachusetts had attracted thousands of tourists from across the country.

The study identified 469 people with COVID-19, 74 per cent of whom were fully vaccinated, following the large gatherings. Testing identified the Delta variant in 90 per cent of virus specimens from 133 people//CNA