“Vaccination is the best tool we have in keeping ourselves and our loved ones safe from the serious disease risk COVID-19 can pose,” Harries said in a statement.

“However, we must also remember that the vaccines do not eliminate all risk: it is still possible to become unwell with COVID-19 and infect others.”

PHE’s findings chime with those from the United States Centers for Disease Control and Prevention (CDC), which last week raised concerns that vaccinated people infected with Delta could, unlike with other variants, readily transmit it.

Vaccines have been shown to provide good protection against severe disease and death from Delta, especially with two doses, but there is less data on whether vaccinated people can still transmit it to others.

“Some initial findings … indicate that levels of virus in those who become infected with Delta having already been vaccinated may be similar to levels found in unvaccinated people,” PHE said.

“This may have implications for people’s infectiousness, whether they have been vaccinated or not. However, this is early exploratory analysis and further targeted studies are needed to confirm whether this is the case.”

The Delta variant has become the dominant form of coronavirus circulating globally, sustaining a pandemic that has already killed more than 4.4 million people, including more than 130,000 in the UK.

According to PHE, it now accounts for 99 percent of all COVID-19 infections in the UK.