Thrombocytopenia Syndrome Rates Following AstraZeneca COVID-19 Vaccine Dose Comparable to Unvaccinated Population


Rates of thrombosis with thrombocytopenia syndrome (TTS), a rare clotting disorder, following a second dose of AstraZeneca’s COVID-19 vaccine (Vaxzevria) are comparable to the rate observed in the unvaccinated population, according to data published in The Lancet. The rates reported are 2.3 per million vaccinees after the second dose and 8.1 per million vaccinees after the first dose.

The analysis used AstraZeneca’s global safety database, which contains all spontaneously reported adverse events from real-world use of its medicines and vaccines worldwide. The cases of TTS reported globally within 14 days of the first or second dose of the vaccine that were included in the study were up to the cut-off date of April 30.

There are currently no known risk factors or definitive causes for TTS following COVID-19 vaccination. According to officials with AstraZeneca, they are currently performing and supporting ongoing investigations of potential mechanisms for this rare adverse event.

“Vaxzevria is effective against all severities of COVID-19 and it plays a critical role in combatting the pandemic,” said Mene Pangalos, PhD, executive vice president of BioPharmaceuticals R&D at AstraZeneca, in a press release. “Unless TTS was identified after the first dose, these results support the administration of the 2-dose schedule of Vaxzevria, as indicated, to help provide protection against COVID-19 including against rising variants of concern.”

AstraZeneca’s vaccine has been granted a conditional marketing authorization or emergency use in more than 80 countries, with over 800 million doses of the COVID-19 vaccine being supplied to more than 170 countries worldwide. It uses a replication-deficient chimpanzee viral vector based on a weakened version of a common cold virus (adenovirus) that causes infections in chimpanzees and contains the genetic material of the SARS-CoV-2 virus spike protein. The surface spike protein is produced after vaccination, priming the immune system to attack the SARS-CoV-2 virus if it later infects the body.


Vaxzevria showed no increased incidence of thrombosis with thrombocytopenia after second dose [news release]. AstraZeneca; July 28, 2021. Accessed July 28, 2021.

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