A new study by the US Centers for Disease Control and Prevention shows 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 without naming the town, suggested the Delta variant of the virus was highly contagious.
The 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.
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 per cent of whom were fully vaccinated, following large public events in the state’s Barnstable County.
Testing identified the Delta variant in 90 per cent of virus specimens from 133 people.
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.
Vaccinated individuals had received one of the three available shots made by Pfizer-BioNTech, Moderna or Johnson & Johnson, the data showed.
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.