First authorized for emergency use in December, AstraZeneca’s viral vector vaccine has been plagued by concerns over rare incidents of blood clots since March.
However, the main takeaway from the study’s authors is that the incidence rates of thrombosis is far higher in people who have been infected by COVID-19 than those who have received either the AstraZeneca or Pfizer/BioNTech.
Comparing adverse events from vaccines to the general population
A pre-print study published in The Lancet this week assessed the incidence rates of blood clotting disorders of thromboembolism and thrombocytopenia, including the very rare thrombosis with thrombocytopenia (TTS) following vaccination with either the Pfizer/BioNTech or AstraZeneca vaccine. This was then compared with expected ‘background’ rates in a general population; and then against the statistics for people with COVID-19.
The study, funded by the European Medicines Agency, covered data from some 6 million people in Catalonia, Spain. Around 1.3 million of these had been vaccinated between December 2020 and May 2021: 945,941 with Pfizer/BioNTech’s BNT162b2 (778,534 with 2 doses) and 426,272 with AstraZeneca’s ChAdOx1.
A further 222,710 people who had been infected with COVID-19 were also included in the study.
“In this study, BNT162b2 [Pfizer] and ChAdOx1 [AstraZeneca] vaccines have been seen to have similar safety profiles,” write the authors in the study. ”Safety signals for both venous thromboembolic events and thrombocytopenia following BNT162b2 vaccination have been identified, with their magnitude similar to these same events among people vaccinated with ChAdOx1. These safety signals must be interpreted with caution with further investigation required to confirm causality.