Russia`s plan to roll-out its "Sputnik-V" Covid-19 vaccine even before completion if all the phases of trials required has sent prompted grave concern among the virus experts, who warn a partially effective shot may encourage the novel coronavirus to mutate.

Viruses, including the pandemic SARS-CoV-2, are known for their ability to mutate all the time - and often this has little or no impact on the risk posed to people.

Russia will begin Covid-19 vaccine trials on 40,000 people from next week.

But some scientists are worried that adding "evolutionary pressure" to the pathogen by deploying what might not be a fully protective vaccine could make things worse.

"Less than complete protection could provide a selection pressure that drives the virus to evade what antibody there is, creating strains that then evade all vaccine responses," said Ian Jones, a virology professor at Britain`s Reading University.

"In that sense, a poor vaccine is worse than no vaccine."

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 skeptical, warning against its use until all internationally approved testing and regulatory hurdles have been passed.