Researchers say they’re making progress in understanding a clotting disorder linked to Covid-19 vaccines that they describe as very rare but potentially “devastating.”

The blood-clotting syndrome affected about 1 in 50,000 people under the age of 50 who received the vaccine developed by AstraZeneca Plc and the University of Oxford, according to a study published in the New England Journal of Medicine. Almost a quarter of those patients who definitely or probably had the condition died.  

The chances of death rose to 73% among those with a very low platelet count and other factors, U.K. researchers found. Cases of the disorder, called immune thrombocytopenia and thrombosis, have declined since age restrictions were introduced in the rollout, scientists said Wednesday.

Researchers hope the findings will help countries that are relying heavily on AstraZeneca’s vaccine to respond to the condition and decide who should receive the shot. The inoculation has been dogged by safety concerns that prompted some regulators to limit its use to older adults. The disorder can affect young, otherwise healthy vaccine recipients.

“What we have learned in the U.K. is hugely important to other countries,” Sue Pavord, a researcher at Oxford University Hospitals, said during a briefing. “If they can recognize this condition and manage it promptly, they can continue with vaccination.”

Data published in July showed AstraZeneca’s vaccine doesn’t raise the risk of the disorder after a second dose. The estimated rate was 2.3 per million in people who received a second shot, comparable to what’s found in an unvaccinated population, but the rate after a single dose was higher, at 8.1 per million.