The prospect of all adults receiving both doses of the Covid-19 vaccine by August is “very optimistic”, England’s chief medical officer said.

Professor Chris Whitty said that while the vaccine rollout was “extraordinarily fast” – with more than 10 million people in the UK having now received their first dose – there were supply constraints.

Speaking at the Downing Street press conference on Wednesday, he said “logistics aren’t about optimism” when he was asked about the timescale of every adult being offered a first dose by May and a second by August.

“I think that May and August strike me as at the very optimistic end,” he said.

“I think if you talk to people who are doing this extraordinarily fast rollout… they would say they will go absolutely hell for leather, as people are doing an incredible job.

“But trying to hit those dates, that is probably beyond what is possible given the constraints of supply and all the other things that we have to deal with.”

Prof Whitty said that those who have received their first dose will soon be due for the second jab, which will “inevitably slow things down” in the rollout.

“But we do have to vaccinate all of them within 12 weeks and that means that from March, we’ll be starting to re-vaccinate as well as the first vaccines, and that will inevitably slow things down,” he said.

He emphasised that it can take several weeks for immunity to build post-vaccination, and that there is still uncertainty over how much the vaccines reduce the risk of transmission.

“They do reduce the risk of severe disease and symptomatic disease of dying and they probably reduce the risk of transmission, and data came out today to support that, but we are not absolutely confident about by how much,” he added.

Infection rates in the community are still “incredibly high”, he warned, as he urged people who have been vaccinated to continue observing social distancing – a point also echoed by the Prime Minister.

It comes after Paul Hunter, professor in medicine at the University of East Anglia (UEA), said social distancing may need to continue until spring 2022 even with effective coronavirus vaccines.

Boris Johnson said those who have had their second dose of the vaccine must stick to social distancing rules, adding that more data on transmission was needed before the Government could consider relaxing restrictions.

He said the focus was on vaccinating everybody as fast as possible and then “taking a view on the interaction between that and the prevalence of the disease”.

He added: “At the moment the level of infection is still forbiddingly high for us to imagine the relaxation of the current guidelines.”