Data suggests that Russia's coronavirus vaccine is 92% effective, according to the country's sovereign wealth fund.
The claims about the Sputnik V vaccine comes days after pharmaceutical giant Pfizer and biotech firm BioNTech released interim results suggesting their vaccine is more than 90% effective at preventing Covid-19.
And three Russian medics who had been given the jab caught coronavirus, raising questions about a drug Vladmir Putin called “world-beating”.
Phase three trials evaluated efficacy among more than 16,000 volunteers who received the vaccine or placebo 21 days after the first injection.
Statistical analysis of 20 confirmed cases of coronavirus, the cases split between vaccinated individuals and those who received the placebo, indicates the vaccine had an efficacy rate of 92% after the second dose.
Covid 19 Time Series
The Russian Direct Investment Fund (RDIF), which has been backing the vaccine, said there were no unexpected adverse events during the trials, and monitoring of the participants is continuing.
The results have not yet been peer-reviewed.
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The phase three study of the vaccine, developed by the Gamaleya Institute, is taking place in 29 clinics across Moscow and will involve 40,000 volunteers in total, with a quarter receiving a placebo shot.
Denis Logunov, deputy director of the Gamaleya Centre, said: "Positive interim results of phase three give reasons to expect a successful outcome of Sputnik V clinical trials.
"We will continue to process and analyse all the data and look to the future with optimism, expecting that results of our work will help end the pandemic sooner."
Eleanor Riley, professor of immunology and infectious disease, University of Edinburgh, said: "I worry that these data have been rushed out on the back of the Pfizer/BioNtech announcement earlier in the week.
"The Sputnik data are based on only 20 cases of Covid-19 in the trial participants, compared to more than 90 cases in the earlier trial.
"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.
"Anything less than this risks a public loss of trust in all vaccines, which would be a disaster."