Britain's surge in coronavirus cases means people in their thirties are now more at risk of severe Covid than getting a blood clot from AstraZeneca's jab.
Spiralling infections caused by the super-infectious Indian 'Delta' variant may mean that the balance of risk is now in favour of vaccination with any of the available jabs.
Officials advised against giving AstraZeneca's jab to under-40s in May when it emerged that the risk of a blood clot was higher than the risk of ending up in intensive care with Covid. This was never the case for older people, whose Covid risk is higher.
When the decision was made the risk of ICU admission for people aged 30 to 39 was just 0.8 per 100,000.
The risk of developing cerebral venous thrombosis (CVT) — an ultra-rare blood clot in the brain — after AstraZeneca's vaccine for people of that age was 1.5 per 100,00, prompting the Government to only offer Pfizer or Moderna jabs to people under 40.
But now the risk of ICU admission with Covid for unvaccinated people in their thirties has more than doubled to 1.9 per 100,000, according to analysis by the Financial Times — meaning being unvaccinated is now riskier than taking the AstraZeneca jab for that age group.
And the current shortage in Pfizer's vaccine could prompt calls to start offering AstraZeneca jabs to young adults again in order to keep the rollout at pace.
Ministers admitted supply of the company's vaccine will be 'tight' over the next month as the country races to vaccinate more people than are becoming infected every day.
Britons in their thirties are now more at risk of Covid than from blood clots caused by AstraZeneca's jab because of the current surge in cases, analysis has shown
Ministers admitted supply of the Pfizer's vaccine will be 'tight' over the next month. (Pictured: People queueing in the rain for their vaccine in London today)
London Mayor Sadiq Khan on Wednesday pleaded for more Pfizer and Moderna vaccines to be sent to the capital to ensure its younger than average population can have a jab soon.
Other Indian variant hotspots across the country have also warned they may not be able to ease restrictions on the delayed 'Freedom Day' on July 19 without more vaccines.
Ian Ward, leader of Birmingham City Council, said: 'Unless the Government can deliver the Pfizer or Moderna vaccines in the right numbers, it's hard to see how we could safely unlock on July 19.'
Cases are surging ahead across the country again, with the more than 11,000 infections counted yesterday the highest daily figure since February.
What is cerebral venous sinus thrombosis (CVT)?
Cerebral venous sinus thrombosis (CVT) is an extremely rare type of blood clot. It may also be abbreviated to CSVT or CVST.
It occurs when the vein that drains blood from the brain is blocked by a blood clot, resulting in potentially deadly bleeding or a stroke.
Symptoms can quickly deteriorate from a headache, blurred vision and faintness to complete loss of control over movement and seizures.
John Hopkins University estimates it affects five in a million people in the US every year, which would suggest 330 patients in Britain suffer from the condition annually.
According to the university, it can affect patients with low blood pressure, cancer, vascular diseases and those prone to blood clotting.
Head injuries can also trigger the condition.
But the JCVI has given no indication yet that it might switch course to jab under-40s with AstraZeneca's vaccine in order to meet demand.
The JCVI 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.'
But a member of the committee told the FT it would be 'perfectly well able to adapt and change' with circumstances.
The committee member said: 'You could get to a place where the risk-benefit balance shifts back because the risk of infection is higher.
'But we remain wary over twisting and weaving too much because that generates public confusion and reduced confidence and also logistic complexity.'
No10 has never publicly released figures detailing exactly how many doses of each of the currently available vaccines are sent to each region, or how many are available in stockpiles.
Health chiefs have bought 100million doses of Pfizer but only 40million are expected this year. Around 24million have already been administered.
Another 17million doses of Moderna's jab have also been purchased — only 500,00 have been dished out.
Despite pressure to go faster to meet the new July 19 target, the national rollout has slowed to under half its peak speed.
Unvaccinated adults in their 20s — who are currently being vaccinated, with the roll-out moving to all adults today — are still less at risk from hospitalisation from Covid (0.8 per 100,000) than they are CVT (1.9) after having an AstraZeneca vaccine.
But their risk has quadrupled since May, when it was 0.2, suggesting it could overtake CVT if cases continue to skyrocket.
And there are also safety fears surrounding Pfizer's vaccine and myocarditis — an ultra-rare heart inflammation — which could prompt a change of approach from No10.
Britain has so far recorded just 34 cases of myocarditis after Pfizer jabs — a similar number to after the AstraZeneca vaccine, suggesting the risk isn't heightened for either of them — and only two after Moderna.
The Medicines and Healthcare products Regulatory Agency (MHRA) says the number of people suffering the condition are 'similar or below expected background levels'.
But the MHRA's equivalent in the US — the Centres for Disease Control and Prevention — has called a meeting to discuss 226 plausible cases of the condition linked to Pfizer's jab.
The MHRA has so far given no indication it believes the threat of myocarditis is large enough to withdraw the use of Pfizer's vaccine.