Researchers have found more evidence for the links between COVID-19 vaccination and a slight increase in the risk of certain adverse health effects, including myocarditis, pericarditis, and Guillain-Barré syndrome.

In the largest COVID-19 vaccine study conducted to date involving nearly 100 million vaccinated individuals across eight countries, researchers from the Global Vaccine Data Network - the research wing of the World Health Organization - evaluated the expected versus observed rates of 13 medical conditions that were considered "adverse events of special interest" in vaccinated individuals. These conditions included Bell's palsy, convulsions, myocarditis, pericarditis, and Guillain-Barré syndrome.

According to the study published in the journal Vaccine, among those who took AstraZeneca's vaccine, there was a 2.5 times higher risk of developing Guillain-Barre syndrome, a rare autoimmune disorder that affects the nerves which could lead to muscle weakness and sometimes, paralysis. Additionally, there was a 3.2 times greater risk of getting blood clots than expected.

The researchers noted an elevated risk of myocarditis - inflammation of the heart muscles - linked to all three doses of Pfizer-BioNTech and Moderna mRNA vaccines, with the highest risk observed in connection to the second dose of the Moderna vaccine.

Regarding pericarditis (inflammation of the outer lining of the heart), a third dose of the AstraZeneca vaccine was linked to a 6.9-fold increased risk, whereas the first and fourth doses of Moderna's vaccine were linked to a 1.7-fold and 2.6-fold increased risk, respectively.

For those who took the Moderna vaccine, the risk of developing acute disseminated encephalomyelitis, a neurological disorder, was 3.8 times greater than expected. On the other hand, there was a 2.2-fold increased risk after AstraZeneca's vaccine.

"This multi-country analysis confirmed pre-established safety signals for myocarditis, pericarditis, Guillain-Barré syndrome, and cerebral venous sinus thrombosis. Other potential safety signals that require further investigation were identified," the researchers wrote.

The researchers cautioned that the study found only associations with adverse health effects and it does not prove that these issues are caused by the vaccines.

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

Approximately 70% of the global population has received at least one dose of a COVID-19 vaccine, indicating that roughly 13.57 billion vaccine doses have been administered worldwide.

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