Topline
People who have received one dose of the AstraZeneca Covid-19 vaccine show an increased rate of rare blood clots when compared with the general population, according to new research in Denmark and Norway, though the researchers emphasized that the vaccine’s overall benefits still vastly outweigh its possible risks.
Some countries have stopped using the AstraZeneca vaccine over blood clot fears.
Key Facts
Using the national health records of some 280,000 people vaccinated with one dose of the vaccine in Denmark and Norway, the researchers compared rates of events like blood clots and heart attacks within 28 days of the shot against the general population.
The researchers identified 59 blood clots in veins in vaccine recipients, 29 more than would have been expected had the group not received shots.
There were higher than expected rates of rare brain blood clots in recipients, the researchers wrote in the British Medical Journal Wednesday, though there was no increase in the rate of arterial clots, heart attacks or strokes.
Whether cases of rare blood clotting in vaccine recipients exceed those in the general population has been a matter of debate.
The researchers wrote that it is important the findings are “interpreted in the light of the proven beneficial effects of the vaccine,” the severity of Covid-19 and the small risks of blood clots.
In a linked editorial, BMJ editors said the AstraZeneca “vaccine is clearly a good choice, despite the likely risks reported in this study," noting that research to tease apart the risks of different vaccines should be a public health priority.
What We Don’t Know
The study was observational, meaning it cannot determine the causes of the blood clots. This is something scientists are still working on and this paper does offer support to there being a link between vaccine and blood clotting.
Crucial Quote
Regulators and experts consistently point to the extremely low risk of developing blood clots versus the serious health implications of a potential Covid-19 infection. In an accompanying op-ed in the BMJ, Professor Paul Hunter, from the University of East Anglia medical school in the U.K., wrote: “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.” The countries that declined to use available AstraZeneca stock at a time of high transmission rates will have “contributed to an increase in the number of avoidable deaths,” he said.
Key Background
A raft of countries halted AstraZeneca vaccine rollouts in early March as a precautionary measure following reports of very rare blood clotting. Though Europe’s medicines regulator ruled out a broad link to blood clotting and described the vaccine as “safe and effective” weeks later, the agency found a “possible link” to rare clotting. Despite this, it still recommended the shot for use, a position shared by the WHO. A number of countries—including France, Germany, the U.K. and Canada—have restricted the shot to older populations, who are believed to be less at-risk of clotting. Denmark and Norway, the subject of this study, have stopped using it entirely. The Johnson & Johnson vaccine is also being investigated amid reports of blood clots, though regulators say the benefits still outweigh the risks.
Further Reading
Denmark Becomes First EU Country To Permanently Block AstraZeneca (Forbes)
AstraZeneca Covid-19 Vaccine Possibly Linked To Rare Blood Clots, European Medicines Agency Says (Forbes)
