Young people could be at a higher risk from rare blood clots after the AstraZeneca coronavirus vaccine because of their more active immune systems, experts have said.

Advice on which vaccine the under-40s in the UK should receive has changed so that healthy people in this age group are to be offered either Pfizer or Moderna as an alternative.

The Joint Committee on Vaccination and Immunisation (JCVI) said that while there is an “extremely small risk” of blood clots after a first dose of the AstraZeneca jab, the chance of younger people falling seriously ill with Covid-19 also reduces as infection rates drop in the UK.

Professor Beverley Hunt, medical director of Thrombosis UK, and professor of thrombosis and haemostasis at King’s College London, was asked whether the reason young people were at more risk of rare clots was due to a more active immune system.

She said: “I think that it’s a reflection of the immune system of young people, compared to older people.

“There’s certainly no other reason to account for it at the moment.”

She said this theory remains speculative as the data is not yet there to back it up, but added: “It’s a good punt.”

Professor Anthony Harnden, deputy chair of the JCVI, agreed, adding: “I would be pretty sure this is related to a vigorous immune response in a very small minority of individuals.”

He noted that this theory is “clearly not the complete story” as clots have occurred in older people too.

The JCVI advises that an alternative jab should be offered to under-40s without underlying health conditions where one is available, and as long as it does not cause any substantial delays to the vaccination programme.

Prof Harnden said: “We fully hope and expect, unless there’s some major problem with vaccine supply or some unusual event which pushes infection rates up considerably, that we will be able to offer every well under-40 an alternative vaccine to AstraZeneca.”

He said “individual circumstances” such as storage difficulties and transport issues could see some being offered AstraZeneca.

Citing the risk of blood clots from the virus, he added: “It’s much better to have the AstraZeneca vaccine than no vaccine at all”.

For those aged just over 40 who are “on the margin” he said the advice is to take whichever jab is offered.

He said: “My strong advice would be ‘stick to our policy advice’.

“If you’re aged 40 and above and you’re offered a vaccine that happens to be AstraZeneca, take it up.

“If you’re aged below 40, and you’re well, then you should be able to be directed to a centre that can give you the choice of an alternative vaccine.”

He said the change in advice was possible due to good supplies and low infection rates.

He said: “If infection rates suddenly soared in this country and we had a large, unvaccinated population under the age of 40, we would be recommending that… we would use this (AstraZeneca) vaccine in addition to other vaccines.”