Arterial events, venous thromboembolism, thrombocytopenia, and bleeding after vaccination with Oxford-AstraZeneca ChAdOx1-S in Denmark and Norway: population based cohort study

SPS commentary:

For cerebral venous thrombosis, standardised morbidity ratio was 20.25 (8.14 to 41.73); an excess of 2.5 (0.9 to 5.2) events per 100 000 vaccinations. The authors advise that these findings should be interpreted in the light of the proven beneficial effects of the vaccine

According to an editorial, it seems quite likely that thromboembolic events were indeed more common among people who had the Oxford-AstraZeneca vaccine, but methodological uncertainties remain. It notes that with all observational studies, critical steps must be taken to minimise potential bias, and in this study, choice of an appropriate control group was one of the most difficult to manage.

It suggests whether the interpretation of the findings are reassuring or concerning depends critically on the type of comparisons. While the rate of venous thromboembolism in vaccinated cohorts was higher than the background rate, all vaccines against covid-19, including the Oxford-AstraZeneca vaccine, reduce mortality from covid-19 substantially. It adds that the absolute magnitude of this benefit varies with the proportion of people exposed to infection over time, but it is known from vaccine trials that mortality reduction far outweighs any risk of adverse events. Furthermore, covid-19 is itself is associated with cerebral venous thrombosis—an estimated 4.3 events per 100 000 infections, which is higher than the 2.5 per 100 000 reported in this study. It notes the need to establish whether the Oxford-AstraZeneca vaccine is unusual in its association with thromboembolic events, or whether other covid-19 vaccines share this characteristic, and also how best to identify those at greatest risk if the association is real.