The companies in a release on Thursday said the booster was tested on 10,000 participants aged 16 and older who had received two doses in its earlier trials.

A booster administered about 11 months after the second shot had a favourable safety profile and worked against the highly contagious Delta variant of the coronavirus, they said.

The data has not been submitted for peer review.

The trial results come a day after the US Food and Drug Administration authorised booster doses US FDA clears Moderna, J&J COVID-19 boosters, backs use of different vaccine for boost of the COVID-19 vaccines from Moderna Inc and Johnson & Johnson, and said Americans could choose a different shot from their original inoculation as a booster.

An panel of expert advisers to the US Centers for Disease Control and Prevention made a similar recommendation on Thursday, clearing the way for a final approval from its director that would allow such shots in the coming days.

The US agencies previously authorised boosters of the Pfizer shot at least six months after the first round of shots to increase protection for people aged 65 and older, those at risk of severe disease and those who are exposed to the virus through their work.

Pfizer had said its two-shot vaccine's efficacy wanes over time, citing a study that showed 84 per cent effectiveness from a peak of 96 per cent four months after a second dose.

Some countries have already gone ahead with plans to give booster doses.

The drug makers said on Thursday the median time between the second dose and the booster shot or the placebo in the study was about 11 months.

The companies said they would submit detailed results of the trial for peer-reviewed publication to the FDA, the European Medicines Agency and other regulatory agencies, as soon as possible.

Australian Associated Press