The findings were revealed by Pfizer and its partner BioNTech as they announced plans to seek regulatory approval for booster jabs.
The data was posted online as part of a company presentation and has not undergone scientific review.
Updated results from the company’s original 44,000-person trial found protection against symptomatic Covid-19 dropped gradually every two months, from a peak of 96 per cent two months after participants got their second dose.
By month four, efficacy was 90 per cent, and by six months it was about 84 per cent.
The presentation also says that protection against the Delta variant was five times higher in adults aged up to 55 who got a third dose of the vaccine. For older age groups, the third dose could provide 11 times more protection against the Delta variant, it said. The claims were based on a much smaller sample.
Separately, New Zealand’s health regulator has granted provisional approval for the Oxford/AstraZeneca vaccine.
The country secured 7.6 million doses of the jab through an advance purchase agreement last year, but so far has only been using the Pfizer/BioNTech vaccine.
Associated Press contributed to this report.
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