Reduced vaccine supplies could slow the easing of lockdown restrictions, an expert has warned.
The NHS said volumes for first doses will be “significantly constrained” from the end of March, leading to a month-long “significant reduction” in weekly supplies.
A letter to local health leaders in England asked vaccination centres and community pharmacy-led services to “ensure no further appointments are uploaded” to booking systems in April.
NHS bosses said that as a result, people under the age of 50 should only get the jab if they are in a priority group for the vaccine.
More than 25 million people in the UK have now received a first dose.
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Dr Simon Clarke, associate professor in Cellular Microbiology at the University of Reading, said while it was not yet known why the delay had occurred, the “ripple effects could last for months”.
“It will undoubtedly make the meeting of the target dates for lifting restrictions more difficult than they otherwise would have been,” he said.
“By pushing back the under-50s first doses, their second doses are also being pushed back.
“If full vaccination becomes required for holidays abroad or even more mundane things like going to the cinema, millions of younger people may end up being excluded from participating for the whole summer.”
Health Secretary Matt Hancock told a Downing Street press conference that vaccine supply was “always lumpy”, but insisted the nation was on track to meet the target of offering a first dose to all over-50s by April 15.
But Labour’s shadow health secretary Jonathan Ashworth urged Mr Hancock to explain what these supply issues were and how the Government was aiming to resolve them.
“Trying to dismiss or downplay the legitimate concerns of anxious people waiting for a vaccine is simply not good enough,” Mr Ashworth added.
Local health leaders have been told to focus efforts on the top priority groups in the NHS letter, signed by Dr Nikita Kanani, medical director for primary care for the NHS in England, and Emily Lawson, chief commercial officer.
The Government’s Vaccine Taskforce “currently predict this will continue for a four-week period, as a result of reductions in national inbound vaccines supply”, the letter said.
It adds: “Those aged 49 years or younger should not be offered vaccination unless they are eligible via a higher cohort because they are e.g., clinically vulnerable, unpaid carer or frontline health and care workers.”
A Pfizer spokeswoman said deliveries “remain on track” for the first quarter of its 40 million dose agreement with the UK, with a “steady supply of vaccines” delivered to the nation.
Meanwhile, an AstraZeneca spokeswoman said: “Our UK domestic supply chain is not experiencing any disruption and there is no impact on our delivery schedule.”
Asked about the letter, Mr Hancock said: “We are committed to all adults being able to get the jab by the end of July and we are on track to deliver on that commitment.”