Plans for a third "booster" coronavirus jab rollout this autumn hang in the balance even though it's due to start in September.
Experts said they are still assessing data before they confirm whether all over-50s and the clinically vulnerable will need a third jab.
They confirmed a booster will "quite likely" be needed for a small number of people who are immunosuppressed.
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There are plans for around 30 million people to receive a third Covid-19 jab alongside a flu vaccine, with the programme due to start on September 6.
But the expert panel that advises the Government, the Joint Committee on Vaccination and Immunisation (JCVI), is still assessing hospital admissions data and blood test samples before they approve the programme.
On Tuesday Health Secretary Sajid Javid said preparations for the booster campaign were ongoing but ministers were awaiting guidance from the clinical experts.
It has been reported that ministers are already planning a 2022 booster campaign after securing 32 million Pfizer doses for autumn next year.
The Times reported that the doses cost £1 billion.
The Department of Health and Social Care (DHSC) said: "We have secured access to more than 500 million doses of Covid-19 vaccines and we are confident our supply will support potential booster programmes in the future."
Asked about a booster campaign for this year, Adam Finn, professor of paediatrics at the University of Bristol, who sits on the JCVI, told BBC Breakfast: "We've been asked to advise as to who might receive a booster if it proves necessary to give boosters.
"I think it's becoming quite clear that there are a small group of people whose immune responses to the first two doses are likely to be inadequate - people who've got immunosuppression of one kind or another, perhaps because they've got immunodeficiency or they've been receiving treatment for cancer or bone marrow transplants or organ transplants, that kind of thing.
"I think it's quite likely we'll be advising on a third dose for some of those groups.
"A broader booster programme is still uncertain - we've laid out potential plans so that the logistics of that can be put together, alongside the flu vaccine programme.
"We need to review evidence as to whether people who receive vaccines early on in the programme are in any serious risk of getting serious disease and whether the protection they've got from those first two doses is still strong. We clearly don't want to be giving vaccines to people that don't need them."
He said it was difficult to say when the JCVI would have more data on whether a wider booster campaign was needed, but experts would be examining hospital admissions and blood tests.
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