AstraZeneca covid-19 vaccine may hinder blood clotting in rare cases

The Oxford/AstraZeneca coronavirus vaccine may be associated with a slightly increased risk of some bleeding disorders, according to new data, but such cases are very rare and the vaccine’s benefits continue to outweigh the risks, say researchers.

An analysis of people who received a first dose of the Oxford/AstraZeneca or Pfizer/BioNTech vaccine revealed a small increased risk of an autoimmune bleeding condition known as immune thrombocytopenic purpura (ITP) associated with the Oxford/AstraZeneca vaccine, separate to the blood clotting syndrome that was linked to the vaccine previously.

In ITP, the immune system destroys platelets, the blood cells that help the blood to clot. The condition can cause minor bruising in some people and excessive bleeding and long-term illness in others, but the symptoms are usually mild and death from ITP is very rare.

It is estimated to occur in about 11 people for every 1 million who receive a first dose of the vaccine.

Aziz Sheikh at the University of Edinburgh, UK, and colleagues say these very small risks are important but rare, and are comparable with those of other jabs, including vaccines against hepatitis B, measles, mumps and rubella, and flu.

They stress that the findings need to be understood within the context of the clear benefits of the Oxford/AstraZeneca vaccine. The risk of developing a serious adverse event related to the vaccine is far lower than the risk of serious illness or death caused by the coronavirus, particularly for older people and other vulnerable populations.

Further research involving younger people and to assess responses to second doses of the vaccines is needed.

“The overall message is that, even if the Oxford/AstraZeneca vaccine does have an increased risk of ITP, its benefit outweighs its risk,” said Stephen Evans at the London School of Hygiene & Tropical Medicine, who wasn’t involved in the study. “For the majority of people ITP does not cause serious problems but it is not the case for everyone.”

The study  examined cases of vaccine-related bleeding and circulatory conditions among 2.53 million adults in Scotland who received their first doses of vaccines between December 2020 and April 2021.

The researchers found that for these people, the Oxford/AstraZeneca jab was associated with a slightly increased risk of ITP up to 27 days after vaccination.

The analysis also found very small increased risks of other arterial blood clot and bleeding events associated with the jab up to 27 days after vaccination.

However, there was insufficient data to determine if there was an association between the Oxford/AstraZeneca vaccine and cerebral venous sinus thrombosis – a rare condition in which a blood clot forms in the brain.

There was no evidence of an increased risk of the adverse events associated with the Pfizer/BioNTech vaccine.

Clinical trials have demonstrated that the Pfizer/BioNTech and Oxford/AstraZeneca vaccines have been generally well tolerated, although there have been a small number of reports of serious side effects.

The UK’s Medicines and Healthcare products Regulatory Agency received 209 reports of bleeding disorders and blood clot disorders after 22 million first doses and 6.8 million second doses of the Oxford/AstraZeneca jab.

Since May, people under 40 in the UK have been offered an alternative vaccine where available, in light of the rare risk of blood clotting syndromes.

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