New study updates evidence on rare blood-clotting condition after COVID-19 vaccination bmj_latest
TTS is currently being investigated as a rare side effect of adenovirus based COVID-19 vaccines, which use a weakened virus to trigger anagainst coronavirus, but no clear evidence exists on the comparative safety of different types of vaccines.
To minimize possible error, participants were matched by age and sex and a range of other potentially influential factors such as pre-existing conditions and medication use were taken into account. The researchers then compared rates of thrombosis and of thrombosis with thrombocytopenia between the adenovirus vaccines and the mRNA vaccines within 28 days after vaccination.
When the data were pooled together, analysis showed a 30% increased risk of thrombocytopenia after a first dose of Oxford-AstraZeneca compared with Pfizer-BioNTech—an absolute risk difference of 8.21 per 100,000 recipients. This is an observational study, and the researchers acknowledge that the rarity of the condition and incomplete vaccine records may have affected the results. What's more, they can't rule out the possibility that some of the observed risk may have been due to other unmeasured factors.