About three quarters of people who contracted the highly contagious coronavirus Delta variant at public events in Cape Cod were fully vaccinated, a study by the US Centres for Disease Control and Prevention (CDC) showed on Friday.
The study found vaccinated people had a similar level of virus presence as those who were 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 the report's findings were a “pivotal discovery” leading to CDC's recommendation this week that masks be worn in areas where cases are 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 in a statement.
Overall, 79 per cent of the vaccinated people who were infected with Covid-19 also reported symptoms such as cough, headache, sore throat and fever.
Four were admitted to hospital, but no one died.
The vaccinated people had received one of the three available shots made by Pfizer-BioNTech, Moderna or Johnson & Johnson, the data showed.
Multiple events in Provincetown, Massachusetts attracted thousands of tourists from across the country, largely for the country's independence holiday on July 4.
A separate CDC internal document, first reported by The Washington Post, described the Delta variant as being as transmissible as chickenpox and said it could cause severe disease.
“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,” Dr Walensky said.
The new study's authors recommended local health authorities to consider requiring masks in indoor public settings regardless of vaccination status or the number of cases in the community “given the potential risk of infection during attendance at large public gatherings that include travellers from many areas with differing levels of transmission".
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.