Oxford and AstraZeneca’s Covid-19 vaccine can be 80 per cent effective when there are three months between shots, an official involved in approving the vaccine in Britain said, but there is insufficient evidence to back a regime involving a half dose.

“Effectiveness was high, up to 80pc, when there was a three month interval between first and second doses, which is the reason for our recommendation,” Munir Pirmohamed, Chair of the Commission on Human Medicines expert Working Group on Covid-19 vaccines, said on Wednesday.

“We also looked at the half dose regimen, which has been publicised quite widely, but we felt that the results were not borne out by the full analysis,” he said at a news conference where the UK’s Medicines and Healthcare products Regulatory Agency (MHRA) presented its decision.