Association of COVID-19 vaccines ChAdOx1 and BNT162b2 with major venous, arterial, or thrombocytopenic events: A...

Area of Science:

• Epidemiology
• Vaccinology
• Public Health

Background:

• Reports of unusual thromboses after COVID-19 vaccination (ChAdOx1-S) prompted investigation into population-level risks.
• The frequency of thrombotic events across different COVID-19 vaccine types requires clarification.

Purpose of the Study:

• To estimate population-level risks of hospitalised thrombocytopenia and major arterial and venous thromboses following COVID-19 vaccination.
• To compare these risks between ChAdOx1-S and BNT162b2 vaccines across different age groups.

Main Methods:

• A whole-population cohort study utilizing linked electronic health records in England (December 2020–March 2021).
• Incidence rates and hazard ratios (HRs) were calculated for thrombotic and thrombocytopenic outcomes 1-28 days and >28 days post-first vaccination dose.
• Analyses were stratified by age (<70 and ≥70 years) and adjusted for multiple covariates, including demographics and medical history.

Main Results:

• Crude incidence rates for venous events were higher post-vaccination compared to pre-vaccination, with slightly higher rates observed after ChAdOx1-S than BNT162b2.
• Crude incidence rates for arterial events were also elevated post-vaccination for both vaccine types.
• Adjusted HRs indicated increased risks for intracranial venous thrombosis (ICVT) and thrombocytopenia 1-28 days post-ChAdOx1-S in those <70 years (aHRs 2.27 and 1.71, respectively), but not significantly after BNT162b2.
• For individuals ≥70 years, rates of arterial or venous thrombotic events were generally lower after either vaccine compared to unvaccinated individuals.

Conclusions:

• Small increases in ICVT and thrombocytopenia were observed after ChAdOx1-S vaccination in adults <70 years, though these risks are minimal compared to COVID-19's morbidity and mortality.
• More precise risk estimates are needed for adults aged <40 years.
• Both COVID-19 vaccines appear suitable for individuals aged ≥70 years, with generally lower thrombotic event rates observed post-vaccination.