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Researchers from NDORMS at the University of Oxford have investigated claims that some adenovirus-based COVID-19 vaccines increase the risk of rare blood clots compared to their mRNA-based counterparts.

Vials of the Covid-19 vaccine © SHUTTERSTOCK

The life-threatening condition known as thrombosis with thrombocytopenia syndrome (TTS) has been seen in a very small number of people after receiving the adenovirus-based Johnson & Johnson or Oxford-AstraZeneca vaccines, but the risk between different COVID-19 vaccines compared to one another remained unclear.

In a new study published in the BMJ, DPhil student Xintong Li and an international team of researchers used routinely collected health data from five European countries and the USA to produced risk estimates. They found that people who received a first dose of the Oxford-AstraZeneca vaccine ChAdOx1 had a 30% increased risk of thrombocytopenia versus people who received a first dose of the mRNA-based BioNTech, Pfizer vaccine. Although requiring further study, they also saw an increased risk of TTS from the Johnson & Johnson Ad26.COV2 vaccine.

Xintong said: “To our knowledge this is the first multinational analysis comparing the safety of COVID-19 vaccines to each other, rather than comparing them to no vaccination. At this stage of the pandemic our study offers an answer to the question of ‘what vaccine is safer’, rather than ‘are vaccines safer than no vaccination?’”

This potential risk needs to be put in context, as COVID infection itself leads to increased thrombosis risk, and vaccination has been shown to reduce COVID-related complications including risk of blood clots.

Ed Burn, Senior Researcher in epidemiology and health economics at NDORMS said: “Cases of any type of blood clot after vaccination are extremely rare overall, however even a small increase over time may result in a substantial number of affected patients due to the large numbers of vaccine doses administered worldwide. The observed risks after adenovirus-based vaccines should be considered when planning further immunisation campaigns and future vaccine development.”

The study was funded by European Medicines Agency.

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