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.
“Regardless of the vaccine used, the increase in rates of thrombosis among persons with COVID-19 is far higher than those seen among persons vaccinated.”
The authors of the study note several limitations. In Spain, fewer people had been vaccinated with AstraZeneca than Pfizer; and this population was younger and healthier individuals (almost all those vaccinated with ChAdOx1 were younger than 70 years old, in accordance with national guidelines).
While distribution of comorbidities and medication was broadly similar across cohorts when stratifying by age categories, vaccine recipients were generally in slightly worse health than the general population.
And thromboembolic events were not able to be split down into particularly rare events of special interest, such as CVST or SVT.
The study breaks down the number of events recorded in vaccination compared to the levels that could be expected in the background population.
"We observed a safety signal for VTE [venous thromboembolism] after a first-dose of BNT162b2, with 46 more events observed than expected among the over 900,000 recipients of this vaccine," they note.
“In general, rates of thrombosis after vaccination with first-dose of ChAdOx1 were similar to those seen after a first-dose of BNT162b2, although with fewer study participants having received this vaccine, there was greater uncertainty around estimates for this vaccine.
“In addition, more cases of thrombocytopenia were seen than were expected following both first and second doses of BNT162b2, while rates of ATE [arterial thromboembolism] were in line with expected rates for both vaccines. Rates of TTS [thrombosis with thrombocytopenia syndrome] were similar to expected for BNT162b2 and fewer than five such events were seen for ChAdOx1. In comparison, rates of VTE and ATE among people with a COVID-19 diagnosis were far higher than background rates.
“While the rate of VTE after vaccination was, for example, around 1.3 fold higher than background rates, it was 8-fold higher after a diagnosis of COVID-19.”