In the largest vaccine safety study to date, researchers from the Global Vaccine Data Network have detected two infrequent side effects associated with COVID-19 vaccines: a neurological disorder and inflammation of the spinal cord.
The study, encompassing data from over 99 million individuals across several countries, including Australia, Argentina, Canada, Denmark, Finland, France, New Zealand, and Scotland, focused on Pfizer, Moderna, and AstraZeneca vaccines.
The analysis, published in the international journal Vaccine, confirmed the already known rare side effects linked to mRNA vaccines (Pfizer and Moderna) and AstraZeneca, such as myocarditis, pericarditis, Guillain-Barré syndrome, and cerebral venous sinus thrombosis.
However, it also revealed a new rare side effect associated with the AstraZeneca vaccine: acute disseminated encephalomyelitis, characterized by inflammation and swelling in the brain and spinal cord.
The findings prompted researchers, including Prof Jim Buttery, co-director of the Global Vaccine Data Network, to conduct a second study independently. Analyzing a dataset of 6.8 million Australians who received the AstraZeneca vaccine, the study confirmed acute disseminated encephalomyelitis as a rare side effect and identified a second new rare side effect – transverse myelitis, or spinal cord inflammation.
Published in Vaccine, the Australian study reported an extremely low risk of acute disseminated encephalomyelitis at 0.78 cases per million doses and 1.82 cases per million doses for transverse myelitis.
Prof Buttery stressed that identifying rare side effects often occurs only after millions of people have received the vaccine, as clinical trials may not have the size to address such issues. He also highlighted that the risk of myocarditis is higher with natural COVID-19 infection than with vaccination, with both conditions being serious but generally recoverable.
Vaccine expert Prof Julie Leask from the University of Sydney emphasized the importance of keeping these findings in perspective. She noted that a COVID-19 infection increases the risk of some of these rare conditions much more than vaccines do. Leask also highlighted the vigilance of vaccine experts in detecting and addressing serious side effects, contributing to a robust vaccination program.
Overall, experts underscored that the benefits of COVID-19 vaccines vastly outweigh the risks, emphasizing the significance of a reliable system for detecting and addressing potential problems.