Pfizer and BioNTech's COVID-19 vaccine is about 91 per cent effective at preventing the disease, they say, citing updated trial data that includes participants inoculated for up to six months.
The shot was also 100 per cent 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 per cent is lower than the 95 per cent 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 drug makers to submit for full US regulatory approval.
The vaccine is currently authorised 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.
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 per cent effective in preventing severe disease as defined by the US Centers for Disease Control and Prevention and 95.3 per cent 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.
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 drug makers to seek US and European approval to use the shot in this age group within weeks.