All participants given a Russian Covid-19 vaccine developed an antibody response and suffered no serious adverse side effects in the early-stage trials, according to results published in The Lancet medical journal.
Experts said the trials were still too small and early to prove the vaccine’s effectiveness and safety – and noted that there had not been a placebo comparison.
The trial took place in two Russian hospitals and involved 76 healthy adults aged 18 to 60.
Each participant was given a dose of the first part of the vaccine and then given a booster with the second part 21 days later.
They were monitored over 42 days and all developed antibodies within the first three weeks.
The vaccine is called Sputnik V, continuing in the naming tradition of the world's first artificial satellite, which was launched into space by the Soviet Union in 1957.
Russian President Vladimir Putin has previously said the vaccine is safe and claimed that one of his daughters had been given it.
But Dr Michael Head of the University of Southampton urged caution.
“This manuscript confirms some of the public statements from a few weeks ago, namely that this appears to be a promising vaccine candidate,” he said.
“At this stage, we do not know if the vaccine actually works – that is what the Phase 3 trials will tell us.
“Public confidence in any licensed vaccine is vital, and suggestions from both Russia and the United States that a vaccine may be fast-tracked without the proper research having taken place are problematic,” he added.
Russia has said it expects to produce between 1.5 million and 2 million doses per month of its potential Covid-19 vaccine by the end of the year, gradually increasing production to 6 million doses per month.
More than half a dozen drugmakers are already conducting advanced clinical trials including industry giants AstraZeneca, Moderna and Pfizer.