People receiving the Covid vaccine made by Oxford-AstraZeneca had a slightly increased risk of a bleeding disorder, and possibly other rare blood problems, researchers reported on Wednesday.

The findings, from a study of 2.53 million adults in Scotland who received their first doses of either the AstraZeneca vaccine or the one made by Pfizer-BioNTech, were published in the journal Nature Medicine. About 1.7 million of the shots were the AstraZeneca vaccine.

The study found no increased risk of the blood disorders with the Pfizer-BioNTech vaccine.

The AstraZeneca vaccine is not authorised for use in the US, but it has been authorised by the European Medicines Agency, the EU’s top drug regulator, as well as many countries outside the bloc. But reports of rare clotting and bleeding disorders in younger adults, some fatal, led a number of countries to limit the vaccine’s use to older people, and a few to drop it altogether.

The new study found that the AstraZeneca vaccine was linked to a slight increase in the risk of a disorder called immune thrombocytopenic purpura, or ITP, which can cause just bruising in some cases but also serious bleeding in others. The risk was estimated at 1.13 cases per 100,000 people receiving their first dose, up to 27 days after vaccination. That estimate would be in addition to the typical incidence in the UK, before the vaccine came into use, which was estimated at six to nine cases per 100,000.

The condition is treatable, and none of the cases in vaccine recipients were fatal, the researchers said. They emphasised that the benefits of the vaccine far outweighed the small risk, and noted that Covid itself is far more likely than the vaccine to cause ITP.

But the researchers also wrote that even though the risks from the AstraZeneca vaccine were small, “alternative vaccines for individuals at low Covid-19 risk might be warranted when supply allows”.

It is not surprising to find ITP in a few vaccine recipients, the researchers said, noting that small increases in the risk have also occurred with the vaccination for measles, mumps and rubella, and the shots for hepatitis B and flu.

In a commentary published with the study, experts on blood disorders said that ITP could be difficult to diagnose and that the possible connection needed further analysis.

But they wrote, “Nonetheless, the risk of vaccination-induced ITP at the rate proposed seems to be far lower than the many risks associated with Covid-19 itself.”

New York Times News Service