The Pfizer-BioNTech coronavirus vaccine has a gradual but steady decline in effectiveness within months of the second dose being taken, a major new study has found.
A vaccinated person's protection against the virus drops to about 84 per cent after four months, still making it highly effective.
Research funded by the US drug maker and German research lab, which studied data from 44,000 vaccinated people, shows protection falls by an average of six per cent every two months.
The vaccine has only been widely used since December 2020, but a projection suggested protection against the virus would still be 50 per cent at 18 months after full vaccination.
The vaccine was 97 per cent effective against severe illness for at least six months, researchers said.
"With up to six months of follow-up and despite a gradually declining trend in vaccine efficacy, BNT162b2 had a favourable safety profile and was highly efficacious in preventing Covid-19," researchers wrote in the summary.
The study has not yet been peer reviewed but was released as Pfizer bids to get approval from US regulators for a third, or booster, dose for vaccinated people, where needed. US authorities have said no booster is required yet.
Pfizer last week also revealed it had already produced a vaccine specially designed to provide protection against the highly contagious Delta variant, which has been linked to a global surge in deaths and cases.
It plans to begin clinical testing soon and seek approval to release the vaccine.
Two doses of Pfizer-BioNTech offers 88 per cent protection against symptomatic disease caused by the Delta variant, compared with 94 per cent against the Alpha variant.
Despite Pfizer's hurdles with US regulators, other countries have begun to give booster shots.
Last week, Israel began to administer third Pfizer doses for people over 60 to protect them against the Delta variant. The UAE also offers third shots of Sinopharm and Pfizer to people who had their second dose three to six months ago.