A rise in coronavirus cases in the UK has put 30 to 39-year-olds at greater risk from severe Covid-19 than from the blood clots associated with the Oxford/AstraZeneca vaccine, casting doubt on official advice that this age group should still be offered an alternative jab.

According to a Financial Times analysis of the latest infections data, in the first week of June the chance of being admitted into intensive care due to Covid-19 over a 16-week period rose to about 1.9 per 100,000 for 30-39 year olds. In comparison, the one-off risk of developing so-called cerebral venous sinus thrombosis due to the AstraZeneca jab was about 1.5 per 100,000.

When the UK government’s Joint Committee on Vaccination and Immunisation in May advised that under-40s should not be offered the AstraZeneca shot because of the blood clot issue, the risk of ICU admission due to Covid-19 for that age group was only 0.8 per 100,000.

The risk calculation does not consider additional benefits from vaccination, such as reducing the likelihood of long Covid, and the risk of severe harm from CVST remains very low.

However the FT analysis suggests that the rise in infections among younger people due to the Delta variant may have tipped the risk-benefit analysis towards using the AstraZeneca jab once again, particularly as some health workers complain of a shortage of other vaccines.

Chart showing that for unvaccinated people aged 30-39 in the UK, the risk of ICU admission for Covid now exceeds the risk of a severe blood clot from the AstraZeneca jab

A total of 11,007 new coronavirus cases were reported on Thursday, the highest since February and a rise of more than 33 per cent over the past week.

The Department of Health and Social Care told the FT it continued to follow the advice given by the JCVI in May.

The JCVI, in response to questions about the FT’s analysis, said: “We have confidence in the government’s current vaccine supply and our advice remains that there is a preference for an alternative to the AstraZeneca vaccine to be offered to those under 40 years of age.”

One JCVI member told the FT that committee would be “perfectly well able to adapt and change”.

“You could get to a place where the risk-benefit balance shifts back . . . because the risk of infection is higher,” the member said. “But we remain wary over twisting and weaving too much because that generates public confusion and reduced confidence, and also logistic complexity.”

The relative risk picture for under-30s has also changed in recent weeks, the FT analysis shows, but the latest data suggest that people aged 20-29 are still better served by taking an alternative coronavirus jab. Their chances of being admitted to intensive care due to Covid-19 is about 0.8 per 100,000, up from 0.2 in April, and their risk of CVST is 1.9 per 100,000.

The shifting picture due to the Delta variant comes as supplies of the BioNTech/Pfizer vaccine, which are intended to vaccinate at least three-quarters of 18 to 39-year-olds, are under strain in some parts of the country.

Prof Kate Ardern, director of public health for Wigan, said two of the seven networks of GPs delivering the vaccine programme in her town would receive no further Pfizer consignments for the next two weeks. “We are not expecting any more. We have been told there is a national shortage,” she said.

A senior public health director in the south of England echoed Ardern’s concerns, saying, “everyone seems short currently”. Whitehall officials and Pfizer said deliveries were taking place exactly as scheduled.

Around 2m 30 to 39-year-olds are yet to receive a first dose of a Covid-19 shot, though hesitancy, health issues and other factors mean not all will be vaccinated.

UK government officials played down the likelihood of any imminent change in the guidance that would see more under-40s receiving the AstraZeneca vaccine. That is despite a growing surplus of AstraZeneca doses that could reach 7m jabs by the end of July, according to estimates by life sciences analytics company Airfinity.

Nadhim Zahawi, the minister in charge of the UK’s coronavirus vaccine rollout, has told MPs the government had sufficient supplies to meet its immunisation targets of fully vaccinating two-thirds of adults, and offering all over-18s at least one shot, by July 19.

People familiar with his remarks said he had warned supply would be “tight, tight, tight with very, very little or no headroom” but that there was enough Pfizer and Moderna — the other vaccine considered suitable for under-40s — to meet the target.

Prof Chris Whitty, England’s chief medical officer, on Thursday said the height of this wave of infections was “still uncertain” and warned of another “winter surge” of coronavirus, even after the current spike in cases has died down.

“We have to just be aware Covid has not thrown its last surprise at us,” he told the NHS Confederation annual conference. “There will be several more over the next period.”

Additional reporting Hannah Kuchler