A new Scandinavian study has confirmed previous data showing increased rates of cerebral venous thrombosis and thrombocytopenia after the AstraZeneca COVID-19 vaccine.
The study also showed higher rates of several thromboembolic and thrombocytopenic outcomes after the Pfizer and Moderna mRNA vaccines, although these increases were less than the rates observed after the AstraZeneca vaccine, and sensitivity analyses were not consistent.
The researchers conclude that confirmatory analysis on the two mRNA vaccines by other methods are warranted.
The study was published in the June issue of JAMA Network Open.
“This study confirms what we know from other studies: that the AstraZeneca vaccine is associated with the rare but serious side effect of vaccine-induced immune thrombotic thrombocytopenia,” lead author Jacob Dag Berild, MD, Norwegian Institute of Public Health, Oslo, told this news organization.
Dr. Dag Berild noted that in the current study there was an excess of 1.6 events of cerebral venous thrombosis per 100,000 AstraZeneca vaccine doses, which is similar to what has been previously reported.
Asked how he saw these results affecting continued use of these vaccines, Dr. Dag Berild pointed out that the risk-benefit ratio of the vaccine depends on the risk of contracting COVID-19 and the risk for a severe outcome from COVID-19 weighed against the risk for an adverse event after vaccination.
“The European Medicines Agency has concluded that the overall risk-benefit ratio remains positive for the AstraZeneca vaccine, but Norway, Finland, and Denmark no longer use the AstraZeneca vaccine in their vaccination programs because of adequate availability of alternative vaccines. I think this is a reasonable decision,” he said.
For the current study, the researchers linked individual-level data separately from national population, patient, and vaccination registers in Norway, Finland, and Denmark. Patient registers were used to identify hospital visits and admissions related to thromboembolic and thrombocytopenic disease in all three countries.
The main outcomes were relative rates of coronary artery disease, coagulation disorders, and cerebrovascular disease in the 28-day period after vaccination, compared with the control period prior to vaccination.
The authors note that a strength of this study is the use of registers with full population coverage in three countries with universal health care, ensuring equal access to care for all permanent residents. At the end of the study period, from Jan. 1, 2020 to May 16, 2021, more than 5.3 million people in the three countries were vaccinated with one or two doses.
Another strength is the inherent adjustment for time-invariant confounders in the self-controlled case series design and the resulting control of confounders that can affect the more traditional observational studies when complete data for confounders are not available, they add.
Of the 265,339 hospital contacts, 43% were made by female patients and 93% by patients born in or before 1971, and 44% were for coronary artery disease, 21% for coagulation disorders, and 35% for cerebrovascular disease.
In the 28-day period after vaccination, there was an elevated rate of coronary artery disease after the Moderna vaccine (relative rate, 1.13) but not after the AstraZeneca (RR, 0.92) or Pfizer (RR, 0.96) vaccines.
There was an observed increase in the rate of coagulation disorders after all three vaccines (AstraZeneca RR, 2.01; Pfizer RR, 1.12; and Moderna RR, 1.26).
There was also an increase in the rate of cerebrovascular disease after all three vaccines (AstraZeneca RR, 1.32; Pfizer RR, 1.09; and Moderna RR, 1.21).
For individual diseases in the main outcomes, two notably high rates were observed after the AstraZeneca vaccine, with relative rates of 12.04 for cerebral venous thrombosis and 4.29 for thrombocytopenia, corresponding to 1.6 and 4.9 excess events per 100,000 doses, respectively.
The elevated risk after the AstraZeneca vaccine was consistent across all three countries and robust in sensitivity analyses.
The researchers report that they also observed statistically significant increases in hospital contacts for thrombocytopenic and thromboembolic events after the Pfizer and Moderna vaccines. However, the risk was smaller than after the AstraZeneca vaccine.
“Additionally, the national estimates varied, increased risk [was] observed only in the oldest cohorts, and sensitivity analysis checking underlying assumptions of the analyses were not consistent. Therefore, the overall and combined increased relative risks following the Pfizer and Moderna vaccinations should be interpreted with caution,” they say.
They note that their results with the AstraZeneca vaccine are in line with a comparison of observed and historic rates performed on partly the same population in Norway and Denmark and also with a Scottish national case-control study.
A version of this article first appeared on.