Under-40s are to be offered an alternative to the Oxford/AstraZeneca coronavirus vaccine.

The Joint Committee on Vaccination and Immunisation (JCVI) said there is an “extremely small risk” of people suffering blood clots after having the jab, but the risk of illness with Covid-19 also drops for younger people as infection rates fall across the country.

While the balance of benefit and risk for the AstraZeneca vaccine is very favourable for older people, it is more finely balanced for younger groups, who do not tend to suffer serious coronavirus illness.

Experts have also assessed the risks from any third wave of Covid in the UK and concluded that that wave is likely to be smaller than previously anticipated, therefore the risks to younger people are lower.

The supply of alternative vaccines, including Pfizer and Moderna, has also meant there can be a change.

It came as an expert source said they anticipate approval of the Johnson & Johnson vaccine in the UK shortly.

The Medicines and Healthcare products Regulatory Agency (MHRA) is currently appraising the jab and the review is said to be at an advanced stage.

Regarding AstraZeneca, the JCVI has advised that another vaccine should be offered to under-40s without underlying health conditions where an alternative is available, and as long as it does not cause any substantial delays to the vaccination programme.

Urging people to accept vaccines, it has warned that reductions in uptake or speed of jab deployment will increase the size of any third wave of infection in the UK.

For this reason, it believes any vaccine would be better than no vaccine, even for those under the age of 40.

However, there are thought to be no supply issues with offering the vast majority of younger people alternatives to AstraZeneca.

Professor Wei Shen Lim, Covid-19 chairman for JCVI, said: “Safety remains our number one priority.

“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.

“The advice is specific to circumstances in the UK at this time and maximises use of the wide portfolio of vaccines available.

“The Covid-19 vaccines have already saved thousands of lives and the benefit for the majority of the population is clear – if you are offered the vaccine, you should take it.”

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 the AstraZeneca vaccine, out of around 28.5 million doses given.

These clots occurred in 141 women and 100 men aged from 18 to 93 (one further case was in a person whose gender was not known), with 49 deaths. Six cases have been reported after a second dose.

A particular type of brain blood clot – cerebral venous sinus thrombosis (CVST) – was reported in 93 cases (with an average age of 47), and 149 had other major thromboembolic events (average age 55) accompanied by low blood platelet count.

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.

Our position remains that the benefits of the Covid-19 vaccine AstraZeneca against Covid-19, with its associated risk of hospitalisation and death, continues to outweigh the risks for the vast majority of people

MHRA chief executive Dr June Raine said: “Public safety is always at the forefront of our minds and we take every report seriously.

“Our position remains that the benefits of the Covid-19 vaccine AstraZeneca against Covid-19, with its associated risk of hospitalisation and death, continues to outweigh the risks for the vast majority of people.

“The balance of benefits and risks is very favourable for older people but is more finely balanced for younger people and we advise that this evolving evidence should be taken into account when considering the use of the vaccine, as JVCI has done.”

In a press briefing, England’s deputy chief medical officer Professor Jonathan Van-Tam was asked about the risk of vaccine hesitancy in light of the new AstraZeneca guidance.

He said: “All of the opinion polls I read very clearly show that the British public has very, very high confidence in the vaccine programme in the UK, can already see what it is doing in terms of changing our future, and the data get better and stronger in terms of the layers of protection, not just the aversion of deaths, serious infections, but now the prevention of transmission on top for young adults.”

Prof Van-Tam said younger people were “young and sociable” and “therefore they have a higher propensity for transmission”, adding that continuing the vaccine programme and maintaining confidence was “critically important”, not only in terms of a third wave, “but basically, because although we’re in a very good epidemiological situation now compared with a couple of months ago even, we don’t retain control of this disease unless we continue the rollout of the vaccination programme”.

On the Johnson & Johnson vaccine, also known as the Janssen vaccine, which has also been linked to a small risk of blood clots, he said it was not thought it would be rolled out in time to complete the UK’s adult vaccine population.

But Prof Van-Tam said because it was a one-shot vaccine “it could potentially be very important, rather than no vaccine, in populations that are hard to reach, basically where you have one chance of vaccinating them and little chance of calling them back for a second vaccine”.

Asked if someone under 40 who turns up for a vaccine and is offered AstraZeneca can re-book to get a different one, he said that is “a bit of a theoretical argument” and he did not expect that to happen.

According to Public Health England (PHE), the vaccine programme is estimated to have prevented more than 10,000 deaths in England alone by the end of March.

Health experts are urging everyone who has had a first dose of AstraZeneca and not suffered a clot to 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.

The same goes for 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.