The vaccine was 100% effective in preventing severe disease as defined by the CDC
Pfizer Inc and BioNTech's Covid-19 vaccine is around 91% effective at preventing the disease, they said on Thursday, citing updated trial data that included participants inoculated for up to six months.
The shot was also 100% effective in preventing illness among trial participants in South Africa, where a new variant called B1351 is dominant, though the number of those participants was relatively small at 800.
While the new overall efficacy rate of 91.3% is lower than the 95% originally reported in November for its 44,000-person trial, a number of variants have become more prevalent around the world since then.
Pfizer Chief Executive Albert Bourla said the updated results, which include data on more than 12,000 people fully inoculated for at least six months, positions the drugmakers to submit for full US regulatory approval.
The vaccine is currently authorized on an emergency basis by the US Food and Drug Administration.
The trial data "provide the first clinical results that a vaccine can effectively protect against currently circulating variants, a critical factor to reach herd immunity and end this pandemic for the global population,” BioNTech Chief Executive Ugur Sahin said in a statement.
Experts fear that new variants of Covid-19 from South Africa and Brazil could be resistant to existing vaccines and treatment. More than 300 cases of the South African variant have been detected in more than 25 US states and jurisdictions, federal data shows.
The vaccine was 100% effective in preventing severe disease as defined by the US Centres for Disease Control and Prevention and 95.3% effective in preventing severe disease as defined by the US Food and Drug Administration.
There were also no serious safety concerns observed in trial participants up to six months after the second dose, the companies said.
They added that it was generally equally effective irrespective of age, race, gender or ethnicity and among participants with a variety of existing medical conditions.
"These data reinforce our view that we have some really potent vaccines," said Danny Altmann, a professor of immunology at Britain's Imperial College London, who was not involved in the Pfizer trial.
He said the effectiveness against the South African variant was "especially noteworthy," since it showed the vaccine is likely to offer effective protection in real-world settings where several different coronavirus variants could be circulating.
The trial reviewed more than 900 confirmed cases of Covid-19, most of which were among participants who received a placebo.
The release of updated results comes on the heels of separate data that showed the vaccine is safe and effective in 12 to 15-year-olds, paving the way for the drugmakers to seek US and European approval to use the shot in this age group within weeks.