Russia's Covid-19 vaccine might actually protect people from catching the disease, early studies show as the country finally publishes data about the controversial jab.
The vaccine became the first in the world to be approved for mass use last month, with Vladimir Putin eyeing up nationwide injections in October.
The move sparked uproar in the scientific community because there was no evidence to prove the vaccine - dubbed Sputnik V - worked or was safe.
But results from two early clinical trials, published today in the prestigious British journal The Lancet, indicate the vaccine is safe and effective.
The Russian scientists behind the studies said the jab stimulated an immune response in all inoculated participants and did not cause any serious health issues.
Production of antibodies seen in the patients suggests the vaccine was able to prepare the body to be able to fend off Covid-19.
Independent Western scientists said the results were 'somewhat reassuring' but warned the trials were too small and narrow to justify injecting millions of Russians.
Just 76 people were involved in the study, only half of whom were actually jabbed, and volunteers were all healthy and mostly in their 20s and 30s.
Vaccines can be more dangerous in elderly and vulnerable because their immune systems are too weak to fight off the tiny amounts of virus in the vaccine.
And just because the jab does not cause harm does not mean it can actually prevent infection in real-life situations. More rigorous research will need to be done to prove this.
Russia's controversial coronavirus vaccine might actually protect people from catching the disease, early clinical data suggests
The vaccine became the first in the world to be approved for mass use last month, with Vladimir Putin eyeing up mass injections in October
Scientists in the US and UK, who were not involved with the work, said the results were 'encouraging' and that the vaccine showed 'promise'.
They were still concerned, however, about the quality of the research and of jumping the gun and pumping the jab into people too soon.
The research was not randomised or placebo-controlled, meaning it was not done to the highest scientific standard and the effects of the jab could not be compared to the health of people who didn't get it.
Randomised controlled trials ensure participants do not know if they are really getting the vaccine or a dud and are deemed critical in removing bias.
The trials took place in two hospitals in Moscow the Burdenko Hospital and Sechenov University Hospital.
The Russian jab is a type called a viral vector vaccine, meaning it uses another virus to carry the immune agent - damaged parts of the real coronavirus, which can trigger a reaction but not cause an infection - into human cells.
Putin's vaccine uses an adenovirus, a type of virus best known for causing the common cold, which has been weakened so it cannot trigger illness.
Oxford University's vaccine candidate uses the same method.
Russia claims the jab sped through early trials on monkeys and humans, known as Phase I and II trials, and was safe and effective at producing antibodies against Covid-19.
But the scientists behind the vaccine have released no scientific data from the trials, meaning the results likely haven't been scrutinised by independent experts.
The Russian jab has also not been put through rigorous Phase 3 trials, which are considered the only way to ensure vaccines are safe and actually work.
During these tests, sometimes known as efficacy trials, scientists give the vaccine to tens of thousands of people and wait to see how many become infected.
They then compare their results with volunteers who caught the infection after receiving a placebo.
Scientists say this final phase is the only way to statistically prove a vaccine prevents infection.
And because it's a much larger testing group, the trials can also pick up subtle side effects that may only affect a small percentage of people.
These rare side effects can become dangerous when vaccines are scaled up for entire populations of tens of millions of people.
Professor Peter Openshaw, an experimental medicine expert at Imperial College London, said today: 'It's important to emphasise that this vaccine has not been approved or even fully tested. The Russian health authorities are discussing the process for possible WHO pre-qualification as an approved vaccine.
'There are currently 19 vaccines that have been tested for the ability to generate antibody (Phase I), another 11 that have passed this stage and gone on to expended testing (Phase II), eight at Phase III and one vaccine approved for limited use.
'So far, it is reported that the Russian vaccine has undergone less than two months of human testing in a total of 38 people. It appears to be at Phase I or II. According to news sources, there is a Phase III trial of 1,600 people planned. That's not actually very large for a vaccine trial and would assume a high rate of infection in the volunteers.'
Professor Openshaw suggested that the type of vaccine the Russians have produced would be likely to cause mild side effects such as fever, headache and tiredness.
Participants were aged between 18 and 60 and all deemed healthy with no underlying health conditions.
In phase 1 of the trial, volunteers were given one part of the vaccine to see if they suffered any negative side effects.
Nearly 60 per cent of participants suffered some pain at their injection site, while half suffered high temperatures - these are generally considered mild, acceptable effects.
Four in 10 reported a sore head, while a quarter felt weak or a lack of energy and 24 per cent had muscle and joint pain.
All of these symptoms were mild and quite common in many other adenovirus vaccines, so the Sputnik V was deemed to be safe and well tolerated.
An adenovirus is a pathogen which is best known for causing common colds and which can be engineered to match the genetic sequence of SARS-CoV-2, the coronavirus that causes Covid-19.
This technique is also being used in the vaccine prototypes developed by Oxford University and China's CanSino.
Phase 2 saw volunteers given the full two-dose vaccine so scientists could monitor their bodies' immune response.
All 40 subjects given the jab produced high levels of antibodies against Covid-19 within 28 days that, in theory, would be enough to fend off the infection.
To analyse the level of immunity each person developed, scientists compared their antibody levels with blood samples taken from patients who had previously been infected with the disease.
The authors say the antibody responses appear to be higher in people vaccinated than those with natural immunity.
The scientists admit their studies were limited, including that they had a short follow-up (42 days), it was a small study, some parts included only male volunteers, and there was no placebo or control vaccine to compare it to.
They say that more research is needed to evaluate the vaccine in different populations, including older age groups, individuals with underlying medical conditions, and people in at-risk groups.
The vaccine will now move into phase 3 trials - which will see tens of thousands of people injected with the vaccine and sent back into the community.
They will be regularly monitored to see if the jab can actually prevent them from being infected with