Britain is facing a shortfall in COVID-19 vaccine supplies that may delay the start of shots for people under 50 after deliveries from two suppliers were curtailed due to production and testing issues.

Prime Minister Boris Johnson described the delays as the inevitable consequence of a complicated program. But he acknowledged that in the short term, the country would receive fewer vaccines than planned a week ago, in part because of a shortfall from India’s Serum Institute.

“That is because of a delay in a shipment from the Serum Institute, who are doing a Herculean job in producing vaccines in such large quantities, and because of a batch that we currently have in the U.K. that needs to be retested as part of our vigorous safety program,”′ he told reporters Thursday at a Downing Street news conference.

Johnson’s comments came a day after the National Health Service told doctors that vaccine supplies would be “significantly constrained” beginning March 29. As a result, people under 50 shouldn’t be inoculated for the time being, unless they have underlying health conditions that put them at higher risk from COVID-19, the NHS said in a letter to public health officials.

The news damped the hopes of the U.K. beginning to vaccinate younger people next month. This next phase of the vaccination program is likely be pushed back until May, said Dr. Martin Marshall, chairman of the Royal College of General Practitioners.

“It was disappointing news when we heard yesterday that the supplies weren’t going to be available during April,” he told the BBC. “It’s a massively successful program overall, and this is a bit of a setback.”

Despite the supply constraints, the government still expects to meet its target of delivering a first vaccine dose to everyone over 50 by mid-April and to all adults by July 31, Health Secretary Matt Hancock said. He also said no vaccination appointments would be canceled and everyone who has had a first shot will get their second shot on schedule.

Britain is using vaccines developed by U.S. drugmaker Pfizer and Anglo-Swedish rival AstraZeneca, with vaccine deliveries from Moderna expected to start soon. More than 25 million people across the U.K. — almost 38% of its population — have received at least one dose of vaccine so far.

The nation’s medicines regulator on Thursday recommended that people continue to receive the AstraZeneca vaccine, despite concerns about blood clots raised in some European countries. The Medicines and Healthcare Products Regulatory Agency said a “rigorous review” of all the data available found no evidence that the AstraZeneca vaccine caused clots in veins.

Another review of five U.K. cases involving a rare type of clot in the brain is under way. This type of clot, which can occur naturally, has been reported in fewer than 1 in every million people vaccinated and no causal link to the vaccine has been established, the MHRA said.

“The benefits of the vaccines against COVID-19 continue to outweigh any risks and that the public should continue to get their vaccine when invited to do so,” the agency said.

Underlining the vaccine’s safety, Johnson told reporters that he would receive the AstraZeneca vaccine on Friday. The prime minister had a serious case of COVID-19 last spring.

“Let’s get the jab done!″ he said.

COVID-19 has killed some 126,000 people in Britain, the highest toll in Europe.

The government had previously said the Serum Institute of India, the world’s biggest vaccine maker, would supply Britain with 10 million doses of the AstraZeneca vaccine this month. But the Serum Institute said Thursday that there were no “stipulated timelines” for delivery of the vaccine.

The company said 5 million doses have already been delivered “and we will try to supply more later, based on the current situation and the requirement for the government immunization program in India.”

India’s foreign ministry says vaccine exports would be calibrated to domestic production, “as well as requirements of our national vaccine program.” COVID-19 is linked to more than 159,000 deaths in India, the world’s fourth-highest death toll.

Dr. Simon Clarke, associate professor of cellular microbiology at the University of Reading, said the disruption in U.K. vaccine supplies is more than just a “bump in the road.” It could have knock-on effects that last for months, including potential delays in lifting COVID-19 restrictions, he said.

Johnson has announced plans to slowly lift a national lockdown in England, though some measures are expected to remain in place until late June. Scotland, Northern Ireland and Wales have also begun to ease restrictions. With the lockdown expected to last past Easter, Britons have been looking at summer holidays as a way to make up for months of living under heavy restrictions.

“By pushing back the under-50s first doses, their second doses are also being pushed back,” Clarke said. “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.”

(AP)