People who received the Covid vaccine, made by Oxford-AstraZeneca, were at a slightly increased risk of developing a bleeding disorder and possibly other rare blood problems, researchers reported Wednesday.

The results of a study of 2.53 million adults in Scotland who received their first dose of the AstraZeneca vaccine or the vaccine obtained from Pfizer-BioNTech were published in Nature Medicine. About 1.7 million of the shots were from the AstraZeneca vaccine.

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

The AstraZeneca vaccine is not approved in the United States, but it has been approved by the European Medicines Agency, the top drug agency in the European Union, as well as many countries outside the bloc. However, reports of rare coagulation and bleeding disorders in younger adults, some of which were fatal, led a number of countries to restrict the use of the vaccine to the elderly and a few to discontinue it altogether.

The new study found that the AstraZeneca vaccine was linked to a slight increase in the risk of a condition called immune thrombocytopenic purpura, or ITP, which can cause bruising in some cases but severe bleeding in others. The risk was estimated to be 1.13 cases per 100,000 people who received their first dose up to 27 days after vaccination. This estimate would be in addition to the typical pre-vaccine incidence in the UK, which has been estimated at six to nine cases per 100,000.

The condition is treatable, and none of the cases in vaccine recipients have been fatal, the researchers said. They stressed that the vaccine’s benefits far outweigh the low risk, noting that Covid itself is much more likely than the vaccine to cause ITP

However, the researchers also wrote that while the risks of the AstraZeneca vaccine are low, “alternative vaccines for those at low risk of Covid-19 may be warranted if supplies allow”.

It wasn’t surprising to find ITP in a few vaccine recipients, the researchers said, noting that the risk also increased slightly with those vaccinated against measles, mumps, and rubella, as well as those vaccinated against hepatitis B and flu.

In a comment published with the study, blood disease experts said ITP could be difficult to diagnose and that the possible association needed further analysis. But they wrote, “Still, the risk of vaccination-induced ITP appears to be far less at the suggested rate than the many risks associated with Covid-19 itself.”

The study in Scotland also found a very small increased risk of arterial clots and bleeding that may be associated with the AstraZeneca vaccine. However, the researchers said there wasn’t enough data to conclude that the vaccine has been linked to a rare type of blood clot in the brain called cerebral venous sinus thrombosis. Earlier this year, reports of these brain clots resulted in some countries suspending or restricting use of the vaccine.

The researchers said they couldn’t rule out a link to the brain clots, but there weren’t enough cases to analyze them.

The brain clots are “as rare as chicken teeth,” said Prof. Aziz Sheikh, lead author of the study from the University of Edinburgh, during a press conference.

Similar concerns have been raised about a rare condition associated with the Johnson & Johnson vaccine, which is approved in the US and other countries, particularly in younger women with brain clots and bleeding. Six U.S. cases, including one fatality, prompted federal health officials to order an interruption in use of the vaccine in April. The break was lifted after 10 days and the vaccine was reinstated with a label to warn consumers of the risk of clots and the availability of other vaccines. Several more cases were later identified and doctors were advised to avoid using heparin, a standard treatment, in these cases as it can make the condition worse.

The risk of clotting has led Denmark to reject the use of the AstraZeneca or Johnson & Johnson vaccines.

AstraZeneca and Johnson & Johnson’s vaccines both use so-called viral vectors to deliver genetic material into the recipient’s cells, and some researchers have suggested that the vectors can lead to the rare blood diseases. It is not known whether there is a connection.

The Scotland study authors said they did not know if their results on the AstraZeneca vaccine had any effect on the Johnson & Johnson vaccine, which they did not study.