The incidence of rare blood clots is higher following a Covid-19 infection than two doses of AstraZeneca or Pfizer/BioNTech vaccines, according to a new study.

The rates of venous thromboembolism (VTE) — a combination of deep vein thrombosis and pulmonary embolism — after the first dose of the AstraZeneca or Pfizer/BioNTech vaccines is 1.3 fold. However, the risk increases 8-fold after diagnosis of Covid-19, revealed the study led and funded by the British-Swedish drugmaker. It is published as a pre-print on The Lancet server.

The study assessed the incidence rates of blood clotting disorders of thromboembolism and thrombocytopenia, including the very rare thrombosis with thrombocytopenia (TTS) following vaccination with a Pfizer/BioNTech mRNA vaccine or AstraZeneca’s Vaxzevria jab, known as Covishield in India, in over one million individuals.

Thromboembolism is the obstruction of a blood vessel by a blood clot that has become dislodged from another site in the circulation, while thrombocytopenia is a low number of platelets in the blood.

The results were then compared with expected rates in a general population and in people with Covid-19.

The findings showed that “safety profiles of AstraZeneca, and the mRNA-based vaccine were similar and overall favourable”.

Very rare clotting disorders (TTS) were observed with both vaccines but these were in line with what would be expected in the general population and lower than in those diagnosed with Covid-19, the study showed.

“This real-world study offers further evidence of the favourable benefit-risk profile of Vaxzeria and demonstrates the critical role all Covid-19 vaccines are playing in combating the pandemic,” said Sir Mene Pangalos, Executive Vice President, BioPharmaceuticals R&D, in a statement.

The analysis included 945,941 mRNA participants (778,534 with two doses), 426,272 AstraZeneca participants, conducted between December 27, 2020 and May 19, 2021. It also included 222,710 Covid-19 participants identified between September 1, 2020 and March 1 2021, and 4,570,149 background participants as of January 1, 2017 from a public health database held in Spain’s Catalonia.

“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 study noted.

Further, these very rare events can be managed when symptoms are identified and treated appropriately, it added.

AstraZeneca Covid jab 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.

After vaccination, the surface spike protein is produced, priming the immune system to attack the SARS-CoV-2 virus if it later infects the body. The vaccine has been granted a conditional marketing authorisation or emergency use in more than 80 countries across six continents, the company said.

More than 800 million doses of Covid-19 Vaccine AstraZeneca have been supplied to more than 170 countries worldwide, including more than 100 countries through the Covax Facility.