Following suspensions by some countries of the AstraZeneca Covid-19 vaccine over suspected blood clots, the UK medicines regulator MHRA has reiterated that the benefits of the vaccine in preventing the disease far outweigh the risks.

People should still go and get their vaccination when asked to do so, the government says, although currently the Joint committee on Vaccination and Immunisation (JCVI) is advising that it is preferable for people under 30 to have a vaccine other than AZ. It warns, though, that they may have to wait if they opt for an alternative vaccine and will be unprotected during that time.

For people who have already had a first dose of AZ vaccine without suffering any serious side effects, the advice is that they should complete the course. This includes people aged 18 to 29 years who are health and social care workers, unpaid carers and family members of those who are immunosuppressed.

Following a rigorous scientific review of the data, the MHRA says that the available evidence does not suggest that venous thromboembolism is caused by the AZ vaccine. This follows a detailed review of reported cases as well as data from hospital admissions and GP records.

Review into blood clot reports underway

However, a further review is underway into a small number of reports from the UK and internationally of a very rare and specific type of blood clot in the cerebral veins – cerebral venous sinus thromboses (CVST) – occurring together with lowered platelets (thrombocytopenia) following the first dose of the AZ vaccine.

CVST occurs when blood clots develop in the cerebral veins occurring together with low platelet counts. These cases are particularly unusual because despite low platelets, there is progressive thrombosis. While the cases reported to date have primarily been venous clots, arterial clots have also been reported.

All cases reported in the UK to date have occurred after the first dose of AZ vaccine. The current data suggests that the overall incidence is around 4 per million first doses of the AZ vaccine administered. The condition is known to occur naturally and is extremely rare. The background rate of CVSTs is estimated to be around 5 to 16 per million annually.

Although cases have been reported in all ages and genders, there appears to be a trend for increasing incidence with decreasing age amongst adults, with the highest incidence reported in the younger adult age groups.

Up to the end of March, the MHRA received 79 reports of thrombosis events with low platelets of which 44 were CVST, out of a total of 20.2 million doses of AZ vaccine given by that date.

Two cases of blood clots (thromboembolism) with thrombocytopenia were reported for the Pfizer/BioNTech vaccine up to and including 31 March, but a review by the MHRA concluded that these were very unlikely to be related to the vaccine. There is currently no evidence to suggest these rare events occur with the Pfizer/BioNTech vaccine.

Further investigations are underway to understand the biological mechanisms and whether the association with the AZ vaccine is related to the vaccine platform or some other immunological mechanism.