The U.K. will offer under-40s an alternative to the AstraZeneca Plc Covid-19 vaccine because of concerns about very rare blood clots that so far appear to be more common in younger people.

The decision, announced Friday, follows a recommendation by U.K. scientific advisory panel the Joint Committee on Vaccination and Immunisation. The precautionary measure comes as authorities prepare to open up the inoculation program to people under 40, after those over 40 were invited to book their first shots in the last week.

The Astra vaccine, created with the University of Oxford, has been the workhorse of the U.K. program to date. The new strategy means that, where supplies permit, people under 40 will be offered vaccines from Pfizer Inc. and BioNtech SE, or Moderna Inc. — the only two other shots that have been approved for use in the U.K. The change won’t impact plans to offer all adults a vaccine by the end of July and is based on the current rate of infection, the prospect of a third wave, and the pace of vaccine rollout.