Lifting lockdown too quickly could trigger a wave of Covid infections with deaths and serious illness concentrated among vaccine hesitant groups of people, a government scientist has warned.

Professor Dame Angela McLean said that while unvaccinated young people might not get “terribly ill”, their infections could “spill over into deaths and serious illness that is very focused in some bits of society.”

This would include vaccine hesitant people who have decided against having a jab, including some ethnic groups like Black, Pakistani and Bangladeshi people who are more at risk of dying.

She said more people need to be vaccinated and suggested new cases need to be low before unlocking, urging Boris Johnson to be driven by “data, not dates”.

McLean said the Scientific Advisory Group for Emergencies (Sage), of which she is a senior member, is “worried” that the UK may not reach herd immunity even when vaccines have been offered to all.

That would risk another wave of infections and it would be “impossible to predict” when that would happen, she told the Commons science committee, echoing warnings from Sage last month.

McLean went on: “A wave amongst the unvaccinated could be very very bad.

“And as we all know there are certain groups in society who are particularly hesitant to take the vaccine and some of those groups are groups that we know seem to suffer disproportionately severe disease when infected.

“So of course that set of people is a real worry.

“None of us want a situation where there is a wave of infection mostly amongst people who are not terribly ill but that spills over into deaths and serious illness that is very focused in some bits of society.”

She added: “I think we can say very, very clearly don’t unlock too fast because if you unlock a lot, whilst a lot of the most vulnerable are still unvaccinated, genuinely we risk disaster.”

Ultimately though, the prime minister must decide what is an acceptable level of infections for the government to tolerate, as eliminating Covid will be impossible, she said.

“I think it’s reasonable to say let’s not have Covid winters that are any worse than bad flu winters,” McLean, the chief scientific adviser at the Ministry of Defence said.

“But bad flu winters can be quite bad. But I’ll be honest with you, it’s something we have cried out for again and again – could somebody in a position of political power tell us what is an acceptable number of infections?”

McLean also said it was “quite unlikely” that society would again operate how it did before the pandemic struck.

“I suspect we just won’t go to work if we have a respiratory illness,” she said.

When asked whether this would be mandated, she added: “It would be most powerful if it became socially unacceptable to go to work with a cough.”