The U.K.’s vaccines advisory body said the Covid-19 vaccine produced by AstraZeneca PLC should preferably not be given to people under 30 following concerns that it causes potentially deadly blood clots in very rare instances.

The decision Wednesday to restrict the vaccine for younger people is a setback for the U.K.’s flagship inoculation drive and leaves the country increasingly dependent on Covid-19 shots developed and produced in other countries.

It also lands another blow on the vaccine developed by AstraZeneca and the University of Oxford, which have faced questions about its efficacy and potential side effects even as tens of millions of doses have been administered following safety signoffs in more than 70 countries world-wide.

The vaccine, which unlike others doesn’t need to be stored at super-cold temperatures, has been regarded as critical to the vaccine rollout in poorer countries, many with relatively young populations.

The speed of the U.K.’s fast-paced vaccine program shouldn’t be affected by the decision as long as vaccine supplies hold up, British officials said. “It is a course-correction, if you like, to the U.K. program,” said U.K. deputy chief medical officer Jonathan Van-Tam.

British Prime Minister Boris Johnson said that he didn’t see “any reason at this stage” to alter the country’s plan to gradually reopen the economy over summer. He added he was confident about the country’s vaccine supply.

Also Wednesday, the European Union’s health agency said it had found possible links between the AstraZeneca vaccine and rare blood clots but that the shot’s benefits continue to outweigh the potential risks. The European Medicines Agency said it found no specific risk factors linking the vaccine to the clots.