Scientists, public health officials and investors welcomed the first successful interim data from a large-scale clinical test as a watershed moment that could help turn the tide of the pandemic if the full trial results pan out. However, mass roll-outs, which needs regulatory approval, will not happen this year and several vaccines are seen as necessary to meet massive global needs.

Pfizer and German partner BioNTech SE said they had found no serious safety concerns yet and expected to seek US emergency use authorization (EUA) this month, raising the chance of a regulatory decision as soon as December.

If granted, the companies estimate they can roll out up to 50 million doses this year, enough to protect 25 million people, and then produce up to 1.3 billion doses in 2021.

"Today is a great day for science and humanity," said Pfizer Chief Executive Albert Bourla, noting the data milestone comes with "infection rates setting new records, hospitals nearing over-capacity and economies struggling to reopen."

Experts said they wanted to see the full trial data, but the preliminary results looked encouraging.

"This news made me smile from ear to ear. It is a relief to see such positive results on this vaccine and bodes well for COVID-19 vaccines in general," said Peter Horby, professor of emerging infectious diseases at the University of Oxford.

There are still many questions, such as how effective the vaccine is by ethnicity or age, and how long it will provide immunity, with social distancing and face covering set to remain for the foreseeable future.

Pfizer expects to seek US emergency use authorization for people aged 16 to 85. To do so, it will need two months of follow-up safety data to assure no side effects crop up. That is expected to be available in the third week of November.

US Health and Human Services Secretary Alex Azar said it would take several weeks for U.S. regulators to receive and process the data before a potential approval.

The prospect of a vaccine electrified world markets with the S&P 500 and Dow hitting record highs as shares of theme park and travel companies battered by the pandemic soared. Movie chain operator AMC Entertainment Holdings was up 60 percent.

Shares in companies that have thrived during lockdowns, such as Netflix Inc and conferencing platform Zoom Video tumbled.

Pfizer shares jumped more than 8% to their highest since July last year, while BioNTech's stock hit a record high.

Mizuho Securities analyst Vamil Divan forecast the vaccine may generate sales in excess of $8.5 billion for Pfizer in 2020-2021 alone.

Shares of other vaccine developers in the final stage of testing also rose with Johnson & Johnson up nearly 4 percent and Moderna Inc, whose vaccine uses a similar technology as the Pfizer shot, up more than 8 percent. Britain's AstraZeneca, however, fell 2 percent. Moderna is expected to report results from its large-scale trial later this month.

"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 in Nashville, Tennessee. "The study isn't completed yet, but nonetheless the data look very solid."

U.S. President Donald Trump welcomed the test results, and the market boost: "STOCK MARKET UP BIG, VACCINE COMING SOON. REPORT 90 percent EFFECTIVE. SUCH GREAT NEWS!" he tweeted.

President-elect Joe Biden said the news was excellent but did not change the fact that face masks, social distancing and other health measures would be needed well into next year.

The World Health Organization called the results very positive, but warned there was a funding gap of $4.5 billion that could slow access to tests, medicines and vaccines in low- and middle-income countries.

There are other challenges as well that could affect less affluent countries. The Pfizer vaccine must be shipped and stored at an extremely cold temperature, which requires necessary infrastructure. That highlights the need for more traditional vaccines in development, such as J&J's candidate.

Still there was cause for jubilation.

"I'm near ecstatic," Bill Gruber, one of Pfizer's top vaccine scientists, said in an interview. "This is a great day for public health and for the potential to get us all out of the circumstances we're now in."

Between 55 percent and 65 percent of the population will need to be vaccinated to break the dynamic of the spread of COVID-19, said Germany's health minister Jens Spahn, adding that he did not expect a shot to be available before the first quarter of 2021.

The European Union said on Monday it would soon sign a contract for up to 300 million doses of the Pfizer and BioNTech COVID-19 vaccine.

The companies have a $1.95 billion contract with the US government to deliver 100 million vaccine doses beginning this year. They did not receive research funding from the Trump administration's Operation Warp Speed vaccine program.

The drugmakers have also reached supply agreements with the United Kingdom, Canada and Japan.

The interim analysis, conducted after 94 participants in the trial developed COVID-19, examined how many had received the vaccine versus a placebo.

Pfizer did not provide that detail, but over 90 percent effective implies that no more than 8 of the 94 had received the vaccine, administered in two shots about three weeks apart.

The efficacy rate, which could drop once full results are available, is well above the 50 percent effectiveness required by the US Food and Drug Administration for a coronavirus vaccine.

Shortly after Pfizer's announcement, Russia said its Sputnik V vaccine was also more than 90 percent effective, based on data collated from inoculations of the public. Its preliminary Phase III trial data is due to be published this month.