A team of researchers from Spain, the United Kingdom and the Netherlands compared data from more than 1.3 million people vaccinated against COVID in Catalonia, looking for evidence of blood clotting disorders that occur are developed after receiving their vaccines.
“In this study including 1,372,213 people vaccinated against SARS-CoV-2, similar safety profiles were observed for the two vaccines,” the researchers wrote in their preprinted article, which is to be published in the Lancet medical journal.
Importantly, the study authors found that people who had been infected with COVID-19 developed blood clots at a much higher rate than those who received any of the AstraZeneca or Pfizer / BioNTech vaccines.
The not-yet-peer-reviewed results come after a difficult time for the AstraZeneca vaccine. In April, the European Medicines Agency (EMA) said there was a “possible link” between the vaccine and “very rare cases of unusual blood clots with low blood platelet counts”. Following the discovery, many countries around the world placed restrictions on its use.
What did the researchers find?
The team, led by researchers from the IDIAP Jordi Gol Institute in Barcelona, found that rates of venous thromboembolism (VTE) – a combination of deep vein thrombosis and pulmonary embolism – were around 1.3 times higher. in people who have received either AstraZeneca or Pfizer / BioNTech. vaccines than in those who had not been vaccinated.
Perhaps more shocking, the EMA-funded study found rates of VTE to be eight times higher in people infected with COVID-19.
Deep vein thrombosis is a condition in which blood clots form in the deep veins of the body, usually in the arms, legs, or groin. Pulmonary embolism (PE) occurs when a blood clot formed by DVT breaks free and is carried to the lungs through the circulatory system, where it blocks all or part of the blood supply. PE can often be fatal.
“VTE rates were higher than expected after the first dose of [Pfizer/BioNTech] and [AstraZeneca], but lower than expected after the second dose of [Pfizer/BioNTech] The researchers wrote.
While they would normally expect to see 62 cases of VTE, the cohort of 222,710 COVID-19 positive patients in the study included 499 cases, an increase of about eight times over normal levels.
The researchers also found that the rates of arterial blood clotting events in recipients of both vaccines were similar or lower than those expected in a similar unvaccinated group.
The rates of thrombocytopenia, or a low platelet count, were higher than expected in those who received a first and second dose of Pfizer / BioNTech, and similar to the levels expected in those who received the AstraZeneca injection.
Commenting on the study, the researchers noted that a difference in the size and demographics of the patient groups receiving each vaccine introduced a degree of uncertainty in the results.
“In general, thrombosis rates after vaccination with a first dose of [AstraZeneca] were similar to those seen after a first dose of [Pfizer/BioNTech], although with fewer study participants receiving this vaccine, there was more uncertainty around the estimates, ”they said.
AZ article reveals no increased risk after second dose
A separate article published in the Lancet on Wednesday by researchers at AstraZeneca found that the rates of a rare blood clotting disorder in people who received both doses of the company’s vaccine were comparable to those in healthy people. and not vaccinated.
According to the AstraZeneca article, the rate of thrombosis with thrombocytopenia syndrome – blood clots with low blood platelet count – was 2.3 per million recipients of the second dose.
However, the study also replicated the finding that blood clots were higher within two weeks of a first dose. “It was 8.1 per million vaccinated after the first dose,” the company said.
The study combined data from 49.23 million people in the EU and UK who had received one dose of the AZ vaccine and 5.62 million who had received both injections by the end of April 2021.