The Government has been urged to “get on” with a coronavirus booster programme rather than waiting for advice from vaccine experts.

Prime Minister Boris Johnson appeared to confirm that a rollout will begin this month, saying older people are the priority as autumn and winter approach.

But the Joint Committee on Vaccination and Immunisation (JCVI) is yet to provide a recommendation on boosters.

The committee’s deputy chairman Professor Anthony Harnden said this week that it is “highly likely” there will be a booster programme, but a final decision has not been made.

He said the committee is awaiting the results of the Cov-Boost study, which is looking at different vaccines to see what immune responses they give and whether jabs can be mixed and matched.

That data is expected next week.

Former health secretary Jeremy Hunt warned that just a few days in a pandemic can make “a big difference”, as he urged politicians to go ahead rather than waiting for the JCVI advice.

He told the BBC Radio 4 Today programme on Friday: “I understand why scientists are taking their time but I think in a pandemic politicians can also read the rooms and see the direction of travel.”

He added: “In a pandemic I think even a few days can make a big difference.

“So I think we should just get on, not wait for that advice, get on with a booster programme.”

Prof Harnden said there is “very complicated modelling and data analysis” going on to decide who should get a booster and when, adding experts do not want to jab people too soon and then be unable to do so again if a new variant emerges.

Professor Saul Faust, chief investigator of the Cov-Boost trial, said it has always been due to report next week, adding that he found it “hard to understand the timing” of comments being made on the topic.

He said: “These data will be available to JCVI and the MHRA (Medicines and Healthcare products Regulatory Agency) next week, immediately after the data arrives from the laboratories and has been formally analysed by the trial team.”

He added that the public “can be reassured that JCVI decisions are the only ones globally being made on hard evidence for a mixed booster schedule”.

On Thursday, Mr Johnson said: “The priorities now are the older generation going into autumn and winter, and we have always said there would be a booster programme in September – in this month – and we are going ahead with that.”

Pressure is also mounting for a decision on jabbing 12 to 15-year-olds, something the JCVI has also not yet recommended.

Meanwhile, an agreement to share vaccine doses with Australia will allow the UK to better align timings of vaccine supply with future need, including for any booster programme or extension of the rollout to younger teenagers, the Department of Health and Social Care said.

Four million doses of the Pfizer jab will be sent from the UK as part of a Covid-19 vaccine deal, with Australia agreeing to return the same “overall volume of doses” before the end of the year, the department said.

The agreement will share doses “at the optimum time to bolster both our countries’ vaccination programmes”, Health Secretary Sajid Javid said.

The first batch of 292,000 doses to Australia is due to be shipped shortly.

Australia has one of the slowest vaccine rollouts among wealthy countries, with just 36.4% of people over the age of 16 fully vaccinated, according to the Australian Immunisation Register.

The deal with Australia comes as the UK announced that the latest batch of its Oxford/AstraZeneca Covid-19 vaccines has been delivered through the Covax scheme, designed to ensure vaccines are available for poorer countries around the world.

The agreement with Australia is separate to the commitment to send 100 million vaccines overseas, the DHSC said.

The latest Government figures show that as of September 1, 72% of the total UK population had received one dose of a coronavirus vaccine.

According to separate figures compiled by Our World in Data, this would place the UK behind countries including the UAE, Spain and Ireland.

Elsewhere, figures from Public Health England show the majority of people in hospital with Covid are under the age of 50 and 73% of those are unvaccinated.

The figures, largely unchanged from the last release a fortnight ago, also show that when it comes to people aged under 50 who died with Covid, 64% of them had not had a jab.