Russia's Sputnik V vaccine is 92% effective at protecting people from COVID-19 according to interim trial results, the country's sovereign wealth fund said on Wednesday, as Moscow rushes to keep pace with Western drugmakers in the race for a shot.

Russia's results are only the second from a late-stage human trial, following on swiftly from data released on Monday by Pfizer Inc (PFE.N) and BioNTech (BNTX.O), which said their shot was also more than 90% effective. 

While experts said the Russian data was encouraging and reinforced the idea the pandemic could be halted by vaccines, they warned that the results were only based on a small number of trial volunteers who had contracted COVID-19.

The analysis was conducted after 20 participants developed the virus and examined how many had received the vaccine versus a placebo. That is significantly lower than the 94 infections in the trial of the vaccine being developed by Pfizer and BioNTech.

"I assume there was political pressure after the press release from Pfizer and BioNTech earlier in the week to now draw level with their own data," said Bodo Plachter, deputy director of the Institute of Virology at the Mainz University. "What is missing for now is an analysis of statistical significance."

To confirm the efficacy rate of its vaccine, Pfizer said it would continue its trial until there were 164 COVID-19 cases.

The Russian Direct Investment Fund (RDIF), which has been backing Sputnik V's development, said the Russian trial would continue for six months.

Alexander Gintsburg, director of the Gamaleya Institute which developed the vaccine, said the interim results demonstrated that Sputnik V was effective and mass vaccinations would be rolled out in Russia in the coming weeks.

European stocks and U.S. stock futures extended their gains slightly after Russia's announcement though the reaction was far more muted than after Pfizer's results.

Successful vaccines are seen as a crucial to restoring daily life around the world by helping end the pandemic that has killed more than 1.26 million people, shuttered businesses and put millions out of work.

However, experts said knowledge about the Russian trial's design was sparse, making it hard to interpret the data.

Scientists have raised concerns about the speed at which Moscow has worked, giving the regulatory go-ahead for the shot and launching mass vaccinations before full trials to test its safety and efficacy had been completed.

"This is not a competition. We need all trials to be carried out to the highest possible standards and it is particularly important that the pre-set criteria for unblinding the trial data are adhered to avoid cherry picking the data," said Eleanor Riley, a professor of immunology and infectious disease at the University of Edinburgh.

"Anything less than this risks a public loss of trust in all vaccines, which would be a disaster."

The results are based on data from the first 16,000 trial participants to receive both shots of the two-dose vaccine.

"We are showing, based on the data, that we have a very effective vaccine," said RDIF head Kirill Dmitriev, adding that it was the sort of news that the vaccine's developers would talk about one day with their grandchildren.

The so-called Phase III trial of the shot is taking place in 29 clinics across Moscow and will involve 40,000 volunteers in total, with a quarter receiving a placebo shot.

The chances of contracting COVID-19 were 92% lower among people vaccinated with Sputnik V than those who received the placebo, the RDIF said.

That's well above the 50% effectiveness threshold for COVID-19 vaccines set by the U.S. Food and Drug Administration.

The RDIF said data from the study would be published in a leading medical journal following a peer review. The results of the early-stage Russian trials were peer reviewed and published in September in The Lancet medical journal.

Experts said that as with the Pfizer results, it was not yet clear how long immunity would last after taking the Russian vaccine, nor how efficient it would be for different age groups.

"We certainly need longer-term observations to draw valid conclusions about efficacy and side effects. The same goes for Pfizer's and BioNTech's numbers," said Plachter in Mainz.

The Russian drug is named Sputnik V after the Soviet-era satellite that triggered the space race, a nod to the project's geopolitical importance for Russian President Vladimir Putin.

Russia registered the vaccine for public use in August, the first country to do so, ahead of the start of the large-scale trial in September.

So far, it has inoculated 10,000 members of the public considered at high risk of contracting COVID-19 such as doctors and teachers, outside of the trial.

The vaccine is designed to trigger a response from two shots administered 21 days apart, each based on different viral vectors that normally cause the common cold: human adenoviruses Ad5 and Ad26.

The Pfizer and BioNTech vaccine uses messenger RNA (mRNA) technology and is designed to trigger an immune response without using pathogens, such as actual virus particles.

Russia is also testing a different vaccine, produced by the Vector Institute in Siberia, and is on the cusp of registering a third, Putin said on Tuesday, adding that all of the country's vaccines were effective.

RDIF said as of Nov. 11 no serious side effects had been reported during the Sputnik V Phase III trial.

Some volunteers had short-term minor adverse events such as pain at the injection site, flu-like syndrome including fever, weakness, fatigue, and headache, it said.

In late October, the vaccination of new volunteers was temporarily paused due to high demand and a shortage of doses.

Russia's deputy prime minister said on Wednesday that the Vector Institute vaccine was expected to post-registration trials on Nov. 15. [nR4N2GK004]

Russia reported 19,851 new coronavirus infections in the past 24 hours and a record high of 432 deaths. At 1,836,960, its overall case tally is the fifth largest in the world, behind the United States, India, Brazil and France.