Five UK medical scientists have criticised a British government plan to delay second doses of Covid-19 vaccines by up to 12 weeks.
They said proven dosing schedules should not be altered "without solid scientific support or evidence".
In an opinion piece published online in the British Medical Journal, the scientists said the plan was based on "assumptions" rather than scientific evidence or trial data.
They also questioned the rationale behind prolonging the time between first and second doses.
The scientists from the universities of Nottingham, Manchester and De Montfort said suggestions by officials on the government's Joint Committee on Vaccines and Immunisation that the delay was caused by shortages of Covid-19 shots were "disputed by vaccine manufacturers".
"The JCVI advice to delay the second dose to between four and 12 weeks is not based on data from the trial, but on an assumption of what would have happened if the second dose hadn't been given at 21 days," they wrote.
"While assumptions can be useful for generating a hypothesis, alone they are not a sufficient reason to alter a known effective dosing regimen."
Britain is distributing two Covid-19 vaccines, one from Pfizer-BioNTech and another from AstraZeneca and the University of Oxford. Both were recently approved for emergency use in the UK.
The vaccine committee last week proposed changing the approved dosing schedules by extending the time before the booster dose.
"Initially vaccinating a greater number of people with a single dose will prevent more deaths and hospitalisations than vaccinating a smaller number of people with two doses," it said.
The proposals have prompted consideration by other governments and generated fierce debate among scientists around the world.
The five scientists, including Herb Sewell, a professor of immunology and consultant immunologist at the University of Nottingham, said stretching the time between doses "could come at increased risk to already high-risk or priority groups".
They urged health authorities to continue allowing second doses of the Pfizer/BioNTech Covid-19 vaccine to be given at the approved three-week interval.
They said it should be continued at least until officials made the data on which their recommendation was based "publicly available for independent scientific review".
'Balance of risk'
England's chief medical officer, Prof Chris Whitty, said on Tuesday that there were unknowns in extending the gap between vaccine doses, and the chance of it leading to a new mutant of the virus was a real worry, but a small one.
Increasing the length of time between doses was a sensible balance of risk, Prof Whitty said.
But the head of the World Health Organisation's immunisation advisory group said on Tuesday that its recommendation for the Pfizer/BioNTech vaccine was two shots within three to four weeks.
BioNTech and Pfizer warned on Monday they had no evidence their vaccine would continue to be protective if the second dose were given more than 21 days after the first.
