A key adviser to the Government on coronavirus has said trials for a vaccine for the disease could be completed by mid-August.

Human testing of a potential vaccine is due to begin within the next week at Oxford University.

Asked about the possibility of a vaccine being produced by the autumn, Professor Sir John Bell, a member of the Government’s vaccine task force and an adviser on life sciences, told BBC Radio 4’s Today programme: “The real question is will it have efficacy?

“Will it protect people, and that has not been tested and it will only be tested once you have vaccinated a significant number of people and exposed them to the virus and counted how many people have got the virus in that population.

“So, we won’t even get a signal for that until May.

“But if things go on course and it does have efficacy, then I think it is reasonable to think that they would be able to complete their trial by mid-August.”

Sir John said of the candidate vaccine being tested at Oxford: “If we can see evidence of a strong immune response by the middle or the end of May, then I think the game is on.

“Then, of course, there is the massive issue of how you manufacture at scale many billions of doses.”

The comments came amid fears that some hospitals could run out of certain types of personal protective equipment (PPE) this weekend.

Chris Hopson, chairman of NHS Providers, said some trusts will run out of supplies on Saturday or Sunday because we have “reached the point where national stock of fully fluid repellent gowns and coveralls (is) exhausted”.

A British Medical Association (BMA) survey of more than 6,000 doctors across the country said a significant amount of them remain without the protection they need to guard against Covid-19.

It echoed another survey of 14,000 medical staff by the Royal College of Nursing (RCN) which found half of nurses have felt pressure to work without appropriate protective equipment during the crisis.

The fears over shortages come after new guidance which means doctors and nurses in England could be asked to work without full-length gowns and to reuse items when treating coronavirus patients provoked a furious backlash from groups representing frontline NHS staff.

Public Health England (PHE) reversed its guidance on Friday evening which stipulated long-sleeved disposable fluid repellent gowns should be worn when treating Covid-19 patients.

If the gowns are not available, clinical staff are now advised to wear “disposable, non-fluid repellent gowns or coveralls” or “washable surgical gowns”, with aprons, and to wash their forearms afterwards.

At least 50 NHS workers have now died after contracting coronavirus.

A Department of Health spokesman said: “New clinical advice has been issued today to make sure that if there are shortages in one area, frontline staff know what PPE to wear instead to minimise risk.”

But Professor Neil Mortensen, president-elect of the Royal College of Surgeons, said: “We are deeply disturbed by this latest change to PPE guidance, which was issued without consulting expert medical bodies. ”

He added: “The new guidance implies that, even in the operating theatre, surgeons and their teams may not require proper PPE. This is simply unacceptable.”

He said that like all doctors, surgeons are committed to their patients and “many will put themselves in the firing line.

“However, if fluid repellent gowns or coveralls are not available, then surgeons should not risk their health.”

That message echoed Royal College of Nursing advice that nurses should refuse to treat patients “as a last resort” if they are not provided with adequate PPE.

Meanwhile, senior Tories have accused ministers of underestimating the public by refusing to discuss exit strategies for ending the coronavirus lockdown

Former Brexit secretary David Davis told the PA news agency: “There are lots and lots of benefits of being open about this.

“The argument that was put, that we don’t want to confuse the message, I think is just wrong.

“It underestimates the public. The public understand that there are phases to this.”