Early results from trials of a Covid vaccine developed in Russia suggest it could be 92% effective.
The data is based on 20 cases of Covid-19 from 16,000 volunteers given the Sputnik V vaccine or a dummy injection.
While some scientists welcomed the news, others said the data had been rushed out too early.
It comes after Pfizer and BioNTech said their vaccine could prevent 90% of people getting Covid-19, based on a study of 43,500 people.
Although the Sputnik data is based on fewer people being vaccinated and fewer cases of Covid developing during the trial, it does confirm promising results from earlier research.
The Sputnik V vaccine, developed at the National Research Centre for Epidemiology and Microbiology in Moscow, is currently going through phase III clinical trials in Belarus, UAE, Venezuela and India.
So far there are no safety issues, with Russian researchers saying there were "no unexpected adverse events" 21 days after volunteers received their first of two injections.
Questions to answer
But there's still a long way to go - this is interim data and, like the Pfizer/BioNTech vaccine trial, the data is still being collected and the full results have not yet been published or scrutinised.
There are questions that still need answering for both vaccines - for example, how well does it protect older people who are most at risk and how long does protection from the vaccine last?
The Russian researchers say their data will be published "in one of the leading international peer-reviewed medical journals".
If it's positive, it means there will almost certainly be more than one way of protecting people against the virus.
Hundreds of vaccines are in development and around a dozen are in the final stages of testing - the Sputnik, Pfizer and Oxford vaccines are three of those.
The Pfizer vaccine injects part of the genetic code of the virus into the body to train the immune system. The other two use a harmless virus that has been genetically modified to resemble the coronavirus.
Two doses are also required, but one advantage of Sputnik is that it doesn't need to be stored at very low temperatures, around -80C, unlike Pfizer's.
'Not a competition'
Alexander Gintsburg, director of Moscow's Gamaleya Research Centre, said Sputnik V would soon be available for a wider population and "lead to an eventual decrease in Covid-19 infection rates, first in Russia, then globally".
Prof Charles Bangham, chair of immunology at Imperial College London, said the results "provide further reassurance that it should be possible to produce an effective vaccine against Covid-19".
However, he added that proper evaluation of the safety and efficacy of both the Russian and Pfizer/BioNTech vaccines was needed when the full data on the trials is published.
Prof Eleanor Riley, from University of Edinburgh, worried the data had been rushed out too soon.
"This is not a competition. We need all trials to be a carried out to the highest possible standards," she said.
The researchers say there have been requests for more than 1.2 billion doses of the Sputnik vaccine from more than 50 countries.
They claim it's possible to produce 500 million doses every year for the global market.