The global rush for a coronavirus vaccine could make the pandemic even worse, say leading experts.
The race is on to immunise the world against Covid-19, which has already killed over 850,000 people, but scientists have warned politicians and commercial companies to be cautious.
Without comprehensive results showing at least 30-50% effectiveness, vaccines risk doing more harm than good, they claim.
On Friday, ministers unveiled a raft of measures designed to fast track the roll-out of any potential coronavirus vaccine in the UK.
The temporary law change could see any vaccine given a green light to be dished out to Brits without going through the usual product approval system.
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Scientists at Oxford developing a vaccine have said they hope to have something ready for regulators to examine before 2021.
While US president Donald Trump wants a vaccine before the presidential election in November.
However, Professor Sir Richard Peto, of Oxford University and an adviser to the World Health Organization, said an initial vaccine with low efficacy would lower the bench mark and lead to even more inferior ones down the line.
He told The Guardian: "I think there’s a big rush, a somewhat nationalistic rush and also somewhat capitalistic rush as well, to be absolutely first to register a vaccine, and it will actually make it more difficult to evaluate other vaccines."
While admitting a vaccine is needed soon, he added: "We really do need quite strong evidence of efficacy."
Downing Street's plan for emergency measures to ensure it could licence a vaccine this year would override any approval from the European Medicines Agency.
Post-Brexit, however, the UK will be free to license its own vaccines and drugs.
Prof Peto is a member of the WHO's Solidarity Vaccines Trial Expert Group, which last week warned a poor vaccine could be worse than none at all.
The group told the Lancet medical journal: "Deployment of a weakly effective vaccine could actually worsen the Covid-19 pandemic if authorities wrongly assume it causes a substantial reduction in risk, or if vaccinated individuals wrongly believe they are immune, hence reducing implementation of, or compliance with, other Covid-19 control measures."
They went on to say they favoured the Solidarity trials, which compare numerous vaccines from around the world as part of a shared control group, rather than individual trials.
The UK government has insisted that any drug will only be passed if it meets "the highest safety and quality standards" and it will put "reinforced safeguards in place".
Student doctors and nurses will join a range of other existing NHS staff in being taught how to administer any vaccine.
If a vaccine is discovered before 2021, the proposals will let it be used before a full product licence is granted, provided it is "proven to be safe and effective during robust and extensive clinical trials".
The Department of Health and Social Care insists that the measures are needed prior to it officially leaving the EU.
