The Government could face some “tough choices” if the vaccine produced by AstraZeneca and Oxford University is found to be less effective than others, a former Department of Health director of immunisation has said.

Professor David Salisbury said ministers would need to think “very carefully” about their handling of the vaccine if its efficacy was lower than for other jabs such as those from Pfizer/BioNTech and Moderna.

The AstraZeneca/Oxford vaccine is currently being assessed by the Medicines and Healthcare products Regulatory Agency (MHRA) for use in the UK.

Data from two arms of its phase-three trial, announced last week, gave a combined efficacy of 70%.

A half dose followed by a full dose was found to be 90% effective in protecting against Covid-19, according to a subset of data, but the figure was 62% for people given two full doses.

The 90% finding was based on a dosing regimen given to 2,741 people, while the two dose arm of the trial involved 8,895 people.

AstraZeneca has said it will carry out a further global clinical trial to assess the 90% dosing regimen, which it acknowledged was as a result of a dosing error.

Prof Salisbury told BBC Radio 4’s Today programme there could be issues if the 90% finding did not hold up under scrutiny.

It comes after some scientists, including in the US, questioned the validity of the data, including the fact the 90% dosing regimen was only on people aged 55 and under.

“I think this has to be unpicked and it may not be possible to do that with the amount of patients that have been recruited so far,” Prof Salisbury said.

“If this vaccine came through at truly 90% and it is a cheaper vaccine and it requires much less rigorous cold chain (storage) than the RNA vaccines (from Pfizer and Moderna), then that would be a great result.

“But if it comes through at 62% and the other vaccines that are coming through so far are 90%, then I think you have to think very carefully what do we do with a 100 million doses of a product that isn’t protecting as well as the alternatives?

“I think there are some tough choices.”

Asked whether the MHRA might say 62% was not good enough when the World Health Organisation (WHO) had set the bar at 50% for an effective vaccine, and AstraZeneca has said the jab protects against serious illness, Prof Salisbury said: “I think they (the MHRA) will judge it on the basis of the criteria that were set, and the information that was given to them and I have great faith in their judgment and their ability to do that.

“I think we will have handling issues about a lower efficacy vaccine when there’s higher efficacy ones available.”

He suggested the most effective vaccines may be prioritised for older and more vulnerable people.

But he added: “The review process that they (the MHRA) do is not necessarily terribly lengthy, it’s the implication that I think has got to be sorted out.

“And the implication is, if this can be a 90% efficacy vaccine, that’s fantastic. If it’s truly 60%, then we’ve got to think very carefully about it.”

A Government scientific adviser said it was too early to make comparisons between vaccine trials.

On Thursday, some British scientists sought to defend Oxford University and AstraZeneca over the clinical trial results, which are expected to be published shortly in The Lancet.

Data from an earlier phase-two study published in The Lancet showed the vaccine produced a strong response in all age groups.

Helen Fletcher, professor of immunology at London School of Hygiene and Tropical Medicine (LSHTM), said: “What’s remarkable is that Oxford University and AstraZeneca have gone from square one to 100 million doses in less than a year.

“It’s not surprising if some manufacturing issues were still being ironed out when they started clinical trials but early stage trials are all about safety and the safety data we have seen has been very robust.”

She said it was important to wait for the full dataset to be published.

“I also think it’s possible that a lower initial vaccine dose could result in higher vaccine efficacy,” she said.

“More is not necessarily better when it comes to vaccines and immunotherapies.

“You need to stimulate an immune response but if you push too hard you can get hit a negative feedback loop where the immune system seeks to control and dampen down the response.

“It’s also possible that a strong immune response to the first vaccine could effectively block an immune response to the second shot of the same vaccine – but we should be able to see evidence of that when the data is published.”

Stephen Evans, professor of pharmacoepidemiology at LSHTM, said: “The idea that no protocol has been published (by Oxford and AstraZeneca) is simply not true.”

He added: “The way the data are put together will have been specified in the protocol and scrutinised very carefully by regulators to ensure that there is no ‘cherry picking’ of the results.”

Professor Andrew Pollard, director of the Oxford Vaccine Group, told reporters at a UK-based press briefing last week that the 90% effectiveness finding had already met the “necessary statistical evidence as required by regulators”.

He said further evidence will probably be available next month but it is “a highly significant result even with the numbers that we have”.

In an interview with the PA news agency last week, Tom Keith-Roach, president of AstraZeneca UK, said there had been “highly statistically significant efficacy results in all subgroups… so I think the regulators will look at it on that basis”.

Oxford University said it would not comment ahead of the data appearing in a peer-reviewed publication.

Meanwhile, Downing Street said there were “no plans” to have the Union flag printed on the Oxford vaccine, following reports that Number 10’s “Union unit” had asked for the British flag to appear on packaging.