No Increased Risk Of Blood Clotting After Second AstraZeneca Shot, Study Finds

Topline

The second shot of AstraZeneca’s Covid-19 vaccine does not increase the risk of developing rare blood clotting, a new study published Wednesday shows, reassuring those who didn’t experience issues after their first shot and following reports of numerous countries offering alternatives.  

Key Facts

The estimated rate of the rare clotting disorder—called thrombosis with thrombocytopenia syndrome, or TTS—following the second AstraZeneca shot was 2.3 per million people vaccinated, according to data published in The Lancet Wednesday.   

This is comparable to the background rates of TTS observed in the general unvaccinated population before Covid-19, indicating there is no elevated risk of developing the disorder in people receiving their second dose.

The data, from AstraZeneca’s global safety database, captured reported cases of TTS in the 14 days after receiving the first or second dose through the end of April 2021, covering some 5.6 million second doses and 49 million first doses.

The data confirmed reports of an elevated risk of TTS in those receiving their first shot, however, which occurred at a rate of 8.1 per million people. 

Sir Mene Pangalos, AstraZeneca’s executive vice president of biopharmaceuticals R&D, said the findings “support the administration of the two-dose schedule… unless TTS was identified after the first dose.”

Key Background 

Despite producing a safe, cheap and effective vaccine at rapid pace, the shot AstraZeneca developed with the University of Oxford has been hit with a number of issues. Originally billed as a world-saver and a more practical alternative to the costly mRNA shots of Pfizer and Moderna, it is now being shunned by wealthy nations who can afford alternatives and poorer nations who don’t have any vaccines at all. Mistakes in clinical trials, concerns about rare blood clots, communications fiascos and frequent trouble delivering promised doses on time all knocked the vaccine’s reputation around the world. Though the risks of developing the rare blood clots are incredibly low—regulators emphasize the risk is vastly outweighed by that of contracting Covid-19—numerous countries have restricted the vaccines to lower-risk groups.    

Crucial Quote

Studies consistently show that the vaccine is highly effective at preventing serious illness, hospitalization and death from Covid-19, including the delta variant. Pangalos described the vaccine as “effective against all severities of Covid-19” and said “it plays a critical role in combatting the pandemic.”

Big Number

399. That’s how many of the more than 49 million people receiving their first shot developed TTS, the study showed. 

Further Reading

Very rare thrombosis with thrombocytopenia after second AZD1222 dose: a global safety database analysis (Lancet)

How AstraZeneca threw away its shot (Politico)

Here’s What Scientists Know About Covid-19 Vaccine Blood Clots, And How The Risks Can Be Diminished (Forbes)

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