Vaccines slightly increase the likelihood of clotting and rare heart and brain conditions, but Covid significantly raises the risk more.
Vaccines against the pandemic coronavirus appear to be associated with rare conditions of the heart, brain, and blood, according to the largest study to date, although experts emphasize that the risk of Covid-19 complications is far greater than the risk of side effects.
Researchers from the Global Vaccine Data Network, a research arm of the World Health Organization, compared the expected and observed frequency of 13 conditions identified as "adverse events of special interest." The analysis covered 99 million vaccinated individuals in eight countries.
As reported in the journal Vaccine, rare cases of myocarditis – inflammation of the heart - were linked to the mRNA vaccines from Pfizer and Moderna. Specifically, Moderna's second dose appears to increase the risk by 6.1 times compared to the condition's frequency in the general population.
However, cases of myocarditis due to vaccination were rare, less than 36 per 100,000 individuals. Researchers also emphasized that Covid-19 infection itself increases the risk more significantly, almost to 65 cases per 100,000.
Another heart condition, pericarditis (inflammation of the thin sac surrounding the heart), increased in frequency by 6.9 times after the third dose of the AstraZeneca vaccine, which is based on viral vector technology. A smaller increase, up to 2.6 times with the fourth dose, was linked to the Moderna vaccine.
The study also links the AstraZeneca vaccine to a 3.2 times increased risk of clotting. However, the coronavirus itself is much more likely to cause clots.
The AstraZeneca vaccine also appears to increase the risk of Guillain-Barre syndrome, a neurological disorder, by 2.5 times. Covid-19, however, increases the risk by 6 times, according to a previous study from 2023.
Recipients of the Moderna vaccine were also found to face a 3.8 times higher risk of developing the rare but neurological disorder of acute disseminated encephalomyelitis, while the AstraZeneca vaccine was linked to a 2.2 times increase.
These findings are in agreement with previous estimates of the risk of side effects and do not change the benefit/risk profile based on which the vaccines were approved.
To date, 13.5 billion vaccine doses have been administered, and 71% of the global population has received at least one dose, according to the Our World in Data service of the University of Oxford.
