A recent study has found that there is no increased risk of developing blood clots after receiving a second dose of AstraZeneca’s coronavirus vaccine and that the rates were comparable to those in healthy, unvaccinated people.
However, this latest study, comparing data from millions of people in the UK and EU who had received the vaccine, found the risk is not increased after the second dose, which could mean a new boost for the confidence in the vaccine.
The study did find that there was a risk following the first shot, after which there were 8.1 reports of the very rare blood clotting disorder, thrombocytopenia syndrome (TTS, or low platelet blood clots) per million vaccinated people, according to data gathered from over 49.23 million people who received one shot.
Following the second shot, however, there were just 2.3 cases of TTS per million people vaccinated based on data gathered from 5.62 million who’d had both jabs as of late April 2021, comparable to the rate in an unvaccinated population.
The study, which has not officially been peer-reviewed, but of which a pre-print was published in the leading medical journal The Lancet, was based on AstraZeneca’s global safety database, which captures all spontaneously reported adverse events from real-world use of its medicines and vaccines worldwide within 14 days of administration of the first or second dose of the vaccine.
“No specific risk factors or definitive cause for TTS following Covid-19 vaccination have been identified and AstraZeneca continues to perform and support ongoing investigations of potential mechanisms,” the company said in a press release on Wednesday.
Increased risk when infected
Another study found that people infected with Covid-19 developed blood clots at a far higher rate than those who had received either of the AstraZeneca or Pfizer/BioNTech vaccines.
The study, funded by the European Medicines Agency (EMA), found that rates of venous thromboembolism (VTE) – a combination of deep vein thrombosis and pulmonary embolism – were eight times higher in people who had been infected with COVID-19.
Overall, the risk of suffering from TTS is higher than expected in those who received a first and second dose of Pfizer/BioNTech and similar to expected levels in those who received AstraZeneca’s shot.
In Belgium, AstraZeneca’s vaccine is only available to people aged 41 and over, and the second dose can be administered starting from eight weeks after a first shot was given.