Pfizer Inc and its German partner BioNTech SE have said their trial COVID-19 vaccine is more than 90 per cent effective, while some experts are urging caution.
The drugmakers are the first to release successful data from a large-scale clinical trial of a coronavirus vaccine.
The companies said they have so far found no serious safety concerns and expect to seek U.S. authorization this month for its emergency use.
If authorized, the number of doses will initially be limited and many questions remain, including how long the vaccine will provide protection.
"Today is a great day for science and humanity," Albert Bourla, Pfizer's chairman and chief executive, said.
"We are reaching this critical milestone in our vaccine development program at a time when the world needs it most with infection rates setting new records, hospitals nearing over-capacity and economies struggling to reopen," Bourla added.
BioNTech Chief Executive Ugur Sahin told Reuters he was optimistic the immunisation effect of the vaccine would last for a year although that was not certain yet.
"The efficacy data are really impressive. This is better than most of us anticipated," said William Schaffner, infectious diseases expert at Vanderbilt University School of Medicine, Nashville, Tennessee. "The study isn't completed yet, but nonetheless the data look very solid."
The prospect of a vaccine electrified world markets with S&P 500 futures hitting a record high and tourism and travel shares surging.
Stocks in European airlines such as ICAG, Lufthansa and AirFrance KLM jumped a third.
Pfizer shares were indicated 12.5 per cent higher in pre-market trading in New York, while BioNTech's U.S. stock leapt 21 per cent.
The company expects to seek broad U.S. authorization for emergency use of the vaccine for people aged 16 to 85. To do so, it will need two months of safety data from about half the study's 44,000 participants, which is expected later this month.
Pfizer and BioNTech have a USD 1.95 billion contract with the U.S. government to deliver 100 million vaccine doses beginning this year. They have also reached supply agreements with the European Union, the United Kingdom, Canada and Japan.
To save time, the companies began manufacturing the vaccine before they knew whether it would be effective. They now expect to produce up to 50 million doses, or enough to protect 25 million people this year.
Pfizer said it expects to produce up to 1.3 billion doses of the vaccine in 2021.
The U.S. pharmaceutical giant said the interim analysis was conducted after 94 participants in the trial developed COVID-19, examining how many of them had received the vaccine versus a placebo.
The company did not break down exactly how many of those who fell ill had received the vaccine. Still, over 90 per cent effectiveness implies that no more than 8 of the 94 people who caught COVID-19 had been given the vaccine, which was administered in two shots about three weeks apart.
The efficacy rate is well above the 50 per cent effectiveness required by the U.S. Food and Drug Administration for a coronavirus vaccine.
To confirm the efficacy rate, Pfizer said it would continue the trial until there are 164 COVID-19 cases among participants. Bourla told CNBC on Monday that based on rising infection rates, the trial could be completed before the end of November.
The data have yet to be peer-reviewed or published in a medical journal. Pfizer said it would do so once it has results from the entire trial.
"These are interesting first signals, but again they are only communicated in press releases," said Marylyn Addo, head of tropical medicine at the University Medical Center Hamburg-Eppendorf in Germany.
"Primary data are not yet available and a peer-reviewed publication is still pending. We still have to wait for the exact data before we can make a final assessment."