It also found those who get infected after receiving two shots of either the Pfizer-BioNTech or the AstraZeneca vaccine may be of greater risk to others than under previous variants of the coronavirus.

Based on more than three million nose and throat swabs taken across Britain, the Oxford University study found that 90 days after a second shot of the Pfizer or Astrazeneca vaccine, their efficacy in preventing infections had slipped to 75 per cent and 61 per cent respectively.

That was down from 85 per cent and 68 per cent, respectively, seen two weeks after a second dose. The decline in efficacy was more pronounced among those aged 35 years and older than those below that age.

"Both of these vaccines, at two doses, are still doing really well against Delta ... When you start very, very high, you got a long way to go," said Sarah Walker, an Oxford professor of medical statistics and chief investigator for the survey.

Walker was not involved in work on AstraZeneca's vaccine, which was initially developed by immunology experts at Oxford.

The researchers would not project how much more the protection would drop over time, but suggested that the efficacy of the two vaccines studied would converge within four to five months after the second shot.

Highlighting the increased risk of contagion from the now prevalent Delta variant, the study also showed that those who get infected despite being fully vaccinated tend to have a viral load similar to the unvaccinated with an infection, a clear deterioration from when the Alpha variant was still dominant in Britain.

The Oxford findings are in line with an analysis by the US Centers for Disease Control and Prevention and come as the US government outlines plans to make COVID-19 vaccine booster shots widely available next month.

Israel began administering third Pfizer doses last month to confront a surge in local infections driven by Delta.

Pfizer has said its vaccine's efficacy drops over time. Last month AstraZeneca said it was still looking into how long its vaccine's protection lasts and whether a booster dose would be needed to keep up immunity.

"The fact that we do see ... more viral load hints (...) that indeed herd immunity might become more challenging," said co-author Koen Pouwels, also of Oxford University.

"Vaccines are probably best at preventing severe disease and slightly less at preventing transmission."

The survey, which has yet to be peer-reviewed before publication in a scientific journal, underscores concerns by scientists that the Delta variant, first identified in India, can infect fully vaccinated people at a greater rate than previous lineages, and that the vaccinated could more easily transmit it.

The study was conducted in partnership with Britain's Office of National Statistics and the Department for Health and Social Care.

Australian Associated Press