The study, by researchers at the University of Southern Denmark and at the Norwegian Institute of Public health, found that those receiving the vaccine faced a one in 40,000 risk of getting a blood clot in the brain, and a one in 9,000 risk of blood clots developing in their veins.
“In the vast majority of countries, it will still overall, no doubt, be net beneficial to use this vaccine, both at the level of society and at the individual level,” Anton Pottegård, a professor at the University of Southern Denmark and the lead author, told The Local.
The researchers reported that their results were “generally reassuring” and those taking the vaccine were at no greater risk than those who had not for most of the side effects they considered.
Pottegård said that the study was the first systematic, bottom-up study looking in detail at all side effects from all of those who were given the AstraZeneca vaccine in Denmark and Norway.
“Until now, people have been dependent on spontaneous reports on the individual patients, and this means a risk of under-reporting of the side effects,” Pottegård said.
He said that although the side effects were too rare to outweigh the benefits in most countries, Denmark’s decision was nonetheless understandable.
“In the Danish setting, I agree there is a case that the benefits do not outweigh the risks,” he said. “We only have fairly young people left for vaccination, a very low rate of transmission, and our healthcare system is not overwhelmed at all, and we have other vaccines incoming.”
He also said that the emerging research on these risks suggested that taking the contraceptive pill, being overweight or having had previous thromboembolic events did not seem to be risk factors.
“This is because this is not predominantly a thromboembolic problem. It’s a problem related to the immune system,” he suggested.
The study was published in the British Medical Journal.