A Covid booster jab will probably be required to protect a small number of the most vulnerable people, but a mass rollout may not be needed, a senior government adviser has said.

Prof Adam Finn, who sits on the Joint Committee on Vaccination and Immunisation (JCVI), said it was still unclear whether all over-50s should be given a third shot after the health secretary, Sajid Javid, said plans were in place to offer the whole cohort a booster.

“We’ve been asked to advise as to who might receive a booster if it proves necessary to give boosters,” Finn told BBC Breakfast on Wednesday.

“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.”

The booster vaccination programme is due to start on 6 September, but the JCVI – the expert panel that advises the government – is still assessing hospital admissions data and blood test samples before it approves the programme.

On Tuesday, Javid said the government was awaiting the JCVI’s advice, but indicated the plan would probably be for over-50s to be offered the booster at the same time as a flu jab. He said those who got their jabs when the vaccine programme began in December last year would be prioritised.

During a visit to a hospital in Milton Keynes, he said he anticipated the rollout beginning in early September and he was making plans for it. He said the vaccine programme had created a “wall of defence”, adding: “It’s massively reduced hospitalisations, deaths from Covid are mercifully low and that’s because of our vaccination programme.”

Finn also said the number of 16- and 17-year-olds becoming “seriously ill” with coronavirus had informed the decision to extend the vaccination rollout to that age group. He said while most young people would have the virus in a mild form, the vaccines would be effective at preventing serious cases.

He told BBC Breakfast: “We’re going cautiously down through the ages now into childhood and it was clear that the number of cases and the number of young people in the age group – 16, 17 – that were getting seriously ill merited going forward with giving them just a first dose.”

On Wednesday, Danny Altmann, a professor of immunology at Imperial College London, said future variants could mean vaccine immunity may never overcome Covid completely.

Asked on Times Radio whether the virus could “die away” once enough people have antibodies, he said: “That’s still kind of true. More vaccination means more people carrying antibodies means fewer susceptible people, which means fewer lungs for the virus to be in, which means less pandemic.

“So, it hasn’t all gone out of the window. But nobody said this virus had to be simple.

“There’s Delta at the moment, there may well be other worse ones coming round the curve, and they impact the effectiveness of the vaccines and change their calculations.”