Two new rare Covid vaccine side effects revealed by global study of over 99 million people

Benefits of vaccination substantially outweigh the risks, scientists say

A global study of over 99 million people across eight countries has identified two new harmful but very rare side effects of Covid-19 vaccines, an advance that could lead to better health monitoring of immunised people.

Researchers part of an international collaboration called the Global Vaccine Data Network (GVDN) hosted at the University of Auckland assessed 13 neurological, blood, and heart-related medical conditions to see if there was a greater risk of them in patients after receiving a Covid-19 vaccine.

The study assessed deidentified data of millions of people who received a Covid-19 vaccine, and examined if there is a greater risk of developing a medical condition in various periods after getting a vaccine compared with before the vaccine became available.

It found that some patients had heart inflammation conditions like myocarditis and pericarditis after they took mRNA vaccines, and some had muscle-weakening Guillain-Barré syndrome and a type of blood clot in the brain after taking viral vector vaccines.

Researchers also found signs of inflammation of part of the spinal cord (transverse myelitis) after taking viral vector vaccines as well as inflammation and swelling in the brain and spinal cord – also known as acute disseminated encephalomyelitis – after some people took viral vector and mRNA vaccines.

However, the chances of having a neurological event after infection with the novel coronavirus were up to 617-fold higher than following Covid-19 vaccination, suggesting that the “benefits of vaccination substantially outweigh the risks,” scientists say.

“This multi-country analysis confirmed pre-established safety signals for myocarditis, pericarditis, Guillain-Barré syndrome, and cerebral venous sinus thrombosis,” scientists wrote, adding that “other potential safety signals” requiring further studies were also identified

“The size of the population in this study increased the possibility of identifying rare potential vaccine safety signals. Single sites or regions are unlikely to have a large enough population to detect very rare signals,” study co-author Kristýna Faksová said in a statement.

Researchers are conducting further studies to build upon the current understanding of Covid-19 vaccines to better unravel their safety using big data.

“By making the data dashboards publicly available, we are able to support greater transparency, and stronger communications to the health sector and public,” Helen Petousis-Harris, another author of the study, said.

While the study identified rare safety signals following Covid-19 vaccination, scientists say “further investigation is warranted to confirm associations and assess clinical significance” of these findings.