Largest multicountry COVID study links vaccines to potential adverse effects

A new study on COVID-19 vaccines that looked at nearly 100 million vaccinated individuals affirmed the vaccines’ previously observed links to increased risks for certain adverse effects including myocarditis and Guillain-Barré syndrome.

The study was conducted by the Global COVID Vaccine Safety project and took into account 99,068,901 vaccinated individuals across eight countries: Argentina, Australia, Canada, Denmark, Finland, France, New Zealand and Scotland.

The report specifically looked at adverse events following administration of the Pfizer, Moderna and AstraZeneca vaccines.

The researchers looked for 13 adverse events of special interest that occurred in vaccine recipients for up to 42 days after shots were administered. These conditions included Guillain-Barré syndrome, Bell’s palsy, convulsions, myocarditis and pericarditis.

Researchers observed a “significant increase” in cases of Guillain-Barré syndrome among those who received the AstraZeneca vaccine with 42 days of administration.

They also noted higher-than-expected instances of acute disseminated encephalomyelitis (ADEM), inflammation of the brain and spinal cord, among those who received their first dose of Moderna’s vaccine.

However, the study noted that when it came to ADEM there was “no consistent pattern in terms of vaccine or timing following vaccination, and larger epidemiological studies have not confirmed any potential association.”

Both mRNA vaccines from Pfizer and Moderna were associated with instances of myocarditis, inflammation of the heart muscle, which occurred more than was expected in the study, with the condition having a significant observed-to-expected ratio consistently after the first, second and third doses.

Significantly higher than expected cases of pericarditis, inflammation of the sac-like structure that surrounds the heart, were also observed following first and fourth doses of Moderna’s vaccine.

“The safety signals identified in this study should be evaluated in the context of their rarity, severity, and clinical relevance,” the researchers wrote.

“Moreover, overall risk–benefit evaluations of vaccination should take the risk associated with infection into account, as multiple studies demonstrated higher risk of developing the events under study, such as GBS, myocarditis, or ADEM, following SARS-CoV-2 infection than vaccination.”

The Global COVID Vaccine Safety project is supported by the Centers for Disease Control and Prevention (CDC) and the Department of Health and Human Services. Several of the authors received financial support from or have relationships with government agencies including the CDC, the New Zealand Ministry of Health and the Canadian Institutes of Health Research, which they disclosed as potential conflicts of interest.

Several of the researchers also reported having relationships or having previously received payments from biopharmaceutical companies Gilead Sciences Inc., AbbVie Inc., Pfizer and GlaxoSmithKline.