Danish-Norwegian registry study on adverse reactions after AstraZeneca vaccination has now been published

​In a Danish-Norwegian collaboration, researchers have shown a slightly increased rate of rare but serious types of cerebral blood clots as well as a generally increased rate of venous blood clots following vaccination with the AstraZeneca vaccine. The risk of such adverse events is considered very low for the individual vaccine recipient.

The new Danish-Norwegian study is the first study to document possible adverse events in relation to the COVID-19 vaccine Vaxzevria from AstraZeneca, in which all vaccine recipients have been followed systematically, as opposed to previous studies, which have relied primarily on reported adverse reactions. 

The new study was a cooperation between Danish and Norwegian research institutions. 

- In this study, we were able to identify all hospital contacts among vaccinated persons by utilising the unique Danish and Norwegian health registers. This ensures that we get a comprehensive of the rate of adverse reactions. Previous studies have been dependent on spontaneous reporting of adverse events in individual patients, which carries a risk of under-reporting, says Anton Pottegård, who is a professor at the University of Southern Denmark and co-author of the study. 

Study results have been continuously disseminated to Danish, Norwegian and international authorities and have already been accounted for in the assessments that have been made. Therefore, it is not expected that the publication of the study results will prompt new assessments by regulators. 

The researchers behind the study stress that their findings do not conflict with the European Medicines Agency’s (EMA) assessment that the vaccine is generally safe. The effects of the vaccine should be seen in the light of the benefits of using the vaccine, which protects against a potentially fatal infection. Depending on how severely affected the individual countries are by the epidemic, the benefits will therefore in many situations outweigh the risks of using the vaccine. 

- The balancing of effects and risks of the vaccine depends on many factors. In many countries, for example those where there are no other vaccines available, where the elderly are not fully vaccinated or where there is a large spread of infection, the benefits of the vaccine will outweigh the risk, both at a societal level and for the individual vaccine recipient, says Anton Pottegård.

Systematic study of 280,000 vaccinated people aged 18-65

The study is based on data from the reputable Danish and Norwegian health registries, which allowed for systematic follow-up of all 280,000 vaccinated people aged 18-65 who have received Vaxzevria in Norway and Denmark. The recipients of the vaccine were followed for 28 days after vaccination regarding hospital contacts in relation to a number of conditions and disorders that could represent adverse events. The incidence of these conditions and disorders was compared with the incidence in the background population.

The study is generally reassuring as researchers have not found an increased incidence for most of the studied adverse events. However, they identified an increased incidence of blood clots in the veins (venous blood clots), including a slightly increased incidence of blood clots in the veins of the brain, corresponding to one additional such clot per 40,000 vaccine recipients. 

These rare blood clots in combination with low platelet counts are referred to as the VITT syndrome. The occurrence of VITT after receiving the AstraZeneca vaccine has led to the removal of the vaccine from the Danish and Norwegian vaccination programmes against COVID-19.

The absolute risk to the individual vaccine recipient is still considered to be low. 

Facts about the study

Contact details

Anton Pottegård
0045 6550 3024
The University of Southern Denmark

Contact information for partners

Prof. Anders Hviid
aii@ssi.dk // 0045 3268 3967
Statens Serum Institut

Prof. Morten Andersen
Morten.andersen@sund.ku.dk // 0045 3533 3506
University of Copenhagen

Assoc. Prof. Reimar W. Thomsen
rwt@clin.au.dk // 0045 8716 8403
Aarhus University

Prof. Øjvind Lidegaard
Oejvind.Lidegaard@regionh.dk // 0045 3545 0950 

Senior Researcher Øystein Karlstad
Oystein.karlstad@fhi.no // 0045 9587 7881
Norwegian Institute of Public Health

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