President Vladimir Putin announced on Tuesday that Russia had become the first country to grant regulatory approval to a COVID-19 vaccine after less than two months of human testing.

Moscow's decision to grant approval before final trials have been completed has raised concerns among some experts.

"It can be dangerous to start vaccinating millions, if not billions, of people too early because it could pretty much kill the acceptance of vaccination if it goes wrong, so I'm very skeptical about what's going on in Russia," Spahn told radio broadcaster Deutschlandfunk.

"I would be pleased if we had an initial, good vaccine but based on everything we know - and that's the fundamental problem, namely that the Russians aren't telling us much - this has not been sufficiently tested," he added.

Spahn said it was crucial, even during a pandemic, to carry out proper studies and tests and make the results public to give people confidence in the vaccine.

"It's not about being first somehow - it's about having an effective, tested and therefore safe vaccine," he said when asked about Russia's vaccine, which will be called "Sputnik V" in homage to the world's first satellite launched by the Soviet Union.

Only about 10% of clinical trials are successful and some scientists fear Moscow may be putting national prestige before safety.

Putin and other officials have said it is completely safe. Government officials have said it will be administered to medical personnel, and then to teachers, on a voluntary basis at the end of this month or in early September. Mass roll-out in Russia is expected to start in October.