Under-40s are to be offered an alternative to the Oxford/AstraZeneca Covid-19 vaccine as a precautionary move.
The Joint Committee on Vaccination and Immunisation (JCVI) has raised the age threshold after the UK's medicines regulator reported new figures on blood clots last week.
Its policy of offering an alternative vaccine previously only applied to the under-30s.
The change comes despite the JCVI and the Medicines and Healthcare products Regulatory Agency (MHRA) saying there are no fresh safety concerns.
The MRHA has said benefits of the vaccine continue to outweigh any risks for the vast majority of people.
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The JCVI, which advises UK health departments on immunisation, has said there is an "extremely small risk" of people suffering blood clots after having the AstraZeneca jab.
But they added that the risk of serious illness with Covid-19 also drops for younger people as infection rates fall across the country.
Professor Wei Shen Lim, Covid-19 chairman for JCVI, said: "We have continued to assess the benefit/risk balance of Covid-19 vaccines in light of UK infection rates and the latest information from the MHRA on the extremely rare event of blood clots and low platelet counts following vaccination.
"As Covid-19 rates continue to come under control, we are advising that adults aged 18 to 39 years with no underlying health conditions are offered an alternative to the Oxford/AstraZeneca vaccine, if available and if it does not cause delays in having the vaccine."
Up to April 28, the MHRA had received 242 reports of blood clots accompanied by low blood platelet count in the UK, all in people who had had the AstraZeneca jab, out of around 28.5 million doses given.
These clots occurred in 141 women and 100 men aged from 18 to 93, and the overall case death rate was 20%, with 49 deaths.
Six cases have been reported after a second dose of the vaccine.
A particular type of brain blood clot - cerebral venous sinus thrombosis (CVST) - was reported in 93 cases and 149 had other major thromboembolic events accompanied by low blood platelet count.
The MHRA said the overall incidence of blood clots with low platelets after a first dose is put at 10.5 per million doses, and about one in a million for a second dose.
For those aged 40 to 49 the incidence is 10.1 per million doses, and 17.4 per million for those aged 30 to 39.
Overall, the death rate per million doses is 2.1, but is 4.5 for those aged 30 to 39.
Health experts have said those who have had a first dose of AstraZeneca and not suffered a clot should have a second dose of the same jab, irrespective of their age.
The MHRA said that, as a precautionary measure, anyone who has a severe headache which is not relieved with painkillers or is getting worse, should seek prompt medical attention at any point from around four days to four weeks after vaccination.
Other side-effects that may need medical attention include a headache that feels worse when a person lies down or bends over, a headache that is unusual and occurs with blurred vision, feeling or being sick, problems speaking, weakness, drowsiness or seizures, a rash that looks like small bruises or bleeding under the skin, and shortness of breath, chest pain, leg swelling or persistent abdominal pain.
Apart from AstraZeneca, the UK is also using two other vaccines - Pfizer/BioNTech and Moderna.
The UK has an in-principle agreement for 60 million doses of the Valneva jab, with an option to acquire a further 130 million doses from 2022-2025.
The country has also ordered 30 million doses of a coronavirus vaccine developed by Johnson & Johnson, which has been shown to be 66 per cent effective in preventing coronavirus infection
Both the Valneva and Johnson & Johnson jabs will need regulatory approval for use in the UK, once data from later-stage trials become available.
