Sputnik-V’s developers, as well as financial backers and Russian authorities, say the vaccine is safe and that two months of small-scale human trials have shown that it works.

But the results of those trials have not been made public, and many Western scientists are sceptical, warning against its use until all internationally approved testing and regulatory hurdles have been passed.

Russia said on Thursday it plans to begin a large-scale efficacy trial of the vaccine in a total of 40,000 people, but will also begin administering it to people in high-risk groups, such as health care workers, before the trial has produced any results.

“You want to make sure the vaccine is effective. We really don’t know that (about the Sputnik vaccine),” said Kathryn Edwards, a professor of paediatrics and vaccine expert in the infectious diseases division at Vanderbilt University School of Medicine in the US.

She said that the risk of what a vaccine might do to a virus — in terms of fighting it, blocking it, or forcing it to adapt — is “always a concern”.

Dan Barouch, a specialist at Harvard’s Beth Israel Deaconess Medical Center in Boston, noted that mutation rates for coronaviruses are far lower than for viruses like HIV, but added: “There are many potential downsides of using a vaccine that doesn’t work. The risk that it (the virus) would mutate is a theoretical risk.”

Scientists say similar evolutionary pressure to mutate is seen with bacterial pathogens, which — when faced with antibiotics designed to target them — can evolve and adapt to evade the drugs and develop resistance.