Russia's "fast-track" Covid-19 vaccine is safe and produces two forms of immune response against the virus, according to a study in the Lancet.
The findings appear to justify some of the claims made by Vladimir Putin in early August, and follow weeks of demands for transparency from the international scientific community.
However, the new study of two early-phase trials reveals that the drug has so far only been tested on 76 people - and not by the most rigorous of methods.
This means it will need to progress a much larger "phase 3" trial, involving thousands of participants, before experts can know how well it works.
UK scientists last night called on Moscow's Gamaleya Centre, where the vaccine is being developed, to resist political pressure to release doses to the general public before all the results are known.
Nevertheless they welcomed data suggesting that the "Sputnik V' candidate stimulates an antibody response, and also a T-Cell response.
Antibodies have been the original focus of many of the prominent vaccine candidates, but virologists have always cautioned that coronavirus antibodies may only last a matter of months and there is no guarantee they prevent infection.
Far less is known about T-cells, but there is some emerging evidence to suggest they do provide good protection and last far longer.
The Russian vaccine is behind other candidates, such as one by Oxford University, which is already conducting a multi-arm international phase 3 trials.
Lasting 42 days, one part of the Gamaleya trial studied a frozen formulation of the vaccine, which is envisaged for large-scale use, while the other analysed a freeze-dried formulation intended for hard-to-reach regions as it is more stable.
The two-part vaccine includes two adenovirus vectors which have been modified to express the Sars-CoV-2 spike protein.
These types of vaccines are based on weakened versions of adenoviruses which are a group of viruses that typically infect membranes of the eyes, respiratory tract, urinary tract, intestines and nervous system, and include the common cold.
The candidates induced an antibody response in all participants within 21 days and a T-cell response within 28 days.
The drug was judged to have a good safety profile at 42 days.
Mild side effects included pain at the injection site, a high temperature and headaches, findings which appear to correspond with President Putin's account of one his daughters who was given the vaccine.
"After the first injection her temperature was 38 degrees, the next day 37.5, and that was it. After the second injection her temperature went up slightly, then back to normal," he told a press conference in August.
The results were published in the latest edition of the Lancet, whose editor Richard Horton has been an outspoken critic of Boris Johnson's handling of the pandemic response.
On Friday, independent experts said results were weakened by the fact the trial was both open-label and non-randomised, meaning participants knew they were getting a vaccine, raising the chance of a placebo effect, and were not chosen at random, meaning doctors might have tried the drug on only the most healthy.
Lead author Dr Denis Logunov, of the NF Gamaleya National Research Centre for Epidemiology and Microbiology, explained that when adenovirus vaccines enter cells, they deliver the Sars-CoV-2 spike protein genetic code, which causes cells to produce the spike protein.
"This helps teach the immune system to recognise and attack the Sars-CoV-2 virus," he said.
"To form a powerful immune response against Sars-CoV-2, it is important that a booster vaccination is provided."
Dr Michael Head, senior research fellow in Global Health at the University of Southampton, said public confidence in the quality of any new vaccine was "vital".
"This manuscript confirms some of the public statements from a few weeks ago, namely that this appears to be a promising vaccine candidate.
Phase 1 and phase 2 trials have been carried out, and there is sufficient reason to scale up into much larger phase 3 trials."
He added: "Concerns do remain around some of the previously-made ambiguous comments that this vaccine is about to be formally approved and licensed.
At this stage, we do not know if the vaccine actually works – that is what the phase 3 trials will tell us."
Professor Eleanor Riley, from the University of Edinburgh, said: "The key question is whether the next step for this vaccine is indeed (as we would expect) a randomised, blinded, placebo controlled phase 3 trial or whether the vaccine developers will come under political pressure to release doses of vaccine for administration to the general public.
"The approval granted for the vaccine – apparently under a law introduced after the onset of the pandemic – allows for both."