Researchers in Norway and Denmark reported Wednesday they had more evidence that AstraZeneca’s coronavirus vaccine might raise the risk of unusual blood clots.
Their study, published in The BMJ medical journal, adds to what’s known about the overall risk, which appears to be small.
The research team studied the medical records of 280,000 people aged 18-65 who received a first dose of the Oxford-AstraZeneca Covid-19 vaccine in Denmark and Norway in February and March. Both countries have extensive national medical registries, making it easy for researchers to use medical records in their studies.
They pulled out reports of heart attacks, strokes, deep vein blood clots and bleeding events that people suffered within 28 days of receiving a first vaccine dose and compared these with expected rates in the general populations of Denmark and Norway.
They found 59 vaccinated people had blood clots in the veins. In that population over that time, 30 would have been expected. This corresponds to 11 excess events per 100,000 vaccinations. “The absolute risks of venous thromboembolic events were, however, small, and the findings should be interpreted in the light of the proven beneficial effects of the vaccine," they wrote. They called the results “reassuring.”
The team did not look for the specific syndrome that has been linked with AstraZeneca’s vaccine as well as the similar vaccine made by Johnson & Johnson’s Janssen vaccine arm. This condition, known as thrombosis with thrombocytopenia syndrome (TTS) or vaccine induced immune thrombotic thrombocytopenia (VITT), is defined by both an unusual type of blood clot and a low count of platelets, which are clot-forming cells in the blood.
They could not find any particular way to identify who might be most at risk of blood clots. They also found people who had been vaccinated were less likely to die from any cause, compared to years past, although they could not explain that and suggested it might be because healthier people were more likely to have been vaccinated.
“The absolute risks described in this study are small in the context of the proven benefits of vaccination against Covid-19, and the globally high incidence of serious cases of SARS-CoV-2 infection,” they concluded.
In a commentary, Dr. Paul Hunter of the University of East Anglia said people should not hesitate to get AstraZeneca’s or Janssen’s vaccine.
“To decline a vaccine today because it is the Oxford-AstraZeneca or Johnson & Johnson vaccine in the hope of being able to get another vaccine sometime later carries a real risk of dying from Covid-19 before being able to get a preferred vaccine. Those countries that delayed their own vaccination programs at a time of high transmission rates by declining to use available Oxford-AstraZeneca vaccines should know that their decision will have contributed to an increase in the number of avoidable deaths from Covid-19,” he wrote.