Researchers who conducted the study stressed, however, that such side effects are extremely rare, and the benefits of the vaccine still outweigh the risks in most situations.
Norway suspended its rollout of the AstraZeneca vaccine on March 11 after a small number of cases of blood clots combined with bleeding and low platelet counts. Denmark has also withdrawn the shot from use. The BMJ study looked at 280,000 18-65 year-olds in Denmark and Norway who had a first dose of the AstraZeneca vaccine between the start of February and March 11.
Results showed 59 blood clots in the veins, compared with 30 expected, equating to 11 excess events for every 100,000 vaccinations. The researchers said this included a higher-than-expected rate of rare brain blood clots, known as cerebral venous thrombosis, at 2.5 cases per 100,000 vaccinations.