Ministers are to be questioned in the house of commons on tuesday over whether they have downgraded plans to vaccinate the entire uk to a new target of less than half the population.

In april, health secretary matt hancock said he was aiming to cover the entire country with a mass programme. i would hope that...given the very positive impact that a vaccine would have, that everybody would have the vaccine, he said.

But on sunday night the financial times revealed that kate bingham, the chair of the uk vaccine task force, believed that less than half the uk population could expect to be vaccinated against coronavirus.

She said that if a successful vaccine was found, inoculating everyone in the country was not going to happen and that we just need to vaccinate everyone at risk.

The new stance threatens to trigger public confusion given mr hancocks previous comment.

Downing street said on monday that if and when we get a vaccine...clearly the priority for vaccines will be the most vulnerable groups.

Jonathan ashworth, shadow health secretary, is expected to raise the issue on tuesday morning in the commons.

The government said it would take advice from the independent joint committee on vaccination and immunisation, although such advice could change based on new information about how the vaccines work.

The committees latest advice, published last month, states that simple age-based programmes are usually easier to deliver and therefore achieve higher vaccine uptake. it places adults over 80, those in care homes, and care home workers at the top of its priority list and then all adults aged 50 to 80, with older age groups first.

Last month, in a podcast produced by the uk government, patrick vallance, the governments chief scientific adviser, highlighted there had only ever been one vaccine, that for smallpox, that had completely eliminated a disease.

He said that for covid-19, its more likely well end up with something that... would mean less severe disease, it would reduce transmission but not stop it completely, and so it would be very effective at maintaining control.

Scientists on tuesday defended ms binghams view. eleanor riley, professor of immunology and infectious disease at edinburgh university, said a policy of vaccinating the most vulnerable made sense if a vaccinereduced the severity of covid-19 but did not block infection or prevent transmission of the virus.

However, if the vaccines that are to be deployed are shown to also reduce transmission, then there is an argument for rolling out the vaccine more widely, she added, noting this would need to be based on a better understanding of the groups in society most responsible for transmission.

Nine months into the pandemic, this is still not well understood, she added.

Stephen evans, professor of pharmacoepidemiology at the london school of hygiene and tropical medicine, said that ms binghams comments were in keeping with what we understand about vaccination.

He noted that while the vaccine might be more effective in younger populations, which is often the case, the risk-benefit analysis of vaccines should be done in absolute rather than relative terms.

He gave the following hypothetical example: if 10 in every 100 older people were to get the disease in the next year, vaccinating all of them with a vaccine that has 50 per cent efficacy would prevent five infections. whereas, if in a younger person the risk of disease is 1 in 1,000, even if the vaccine efficacy was as great as 80 per cent, only 0.8 lives out of a 1,000 would be saved by vaccination.

It is perfectly reasonable to start out giving the vaccine to those who are most likely to have the most benefit, he added.

The government has secured early access to 360m vaccine doses through agreements with six separate pharmaceutical companies.

Ms bingham has previously said there would be limited supply of any available vaccines until about midway through 2021, and has reiterated several times that our starting point is we think many of these vaccines will fail.

By spreading our bets were in a much stronger position, she said.