Large study funded by CDC identifies rare COVID-19 vaccine risks

A new study confirmed a slightly increased risk of several conditions following COVID-19 vaccination.
A healthcare worker fills a syringe with the Pfizer COVID-19 vaccine at Jackson Memorial Hospital on Oct. 5, 2021.
Posted at 9:36 AM, Feb 20, 2024

COVID-19 vaccines have been available to the public for nearly three years, and researchers have been gathering information on their widespread use. 

Recently, a multinational group of researchers published their findings on the potential risks and side effects of the leading COVID-19 vaccines. The Global COVID Vaccine Safety (GCoVS) Project was supported by the Centers for Disease Control and Prevention, which provided over $10 million for the study. 

In what was dubbed as the largest such study on COVID-19 vaccine safety, the project confirmed that vaccinated people faced slightly increased risks of several medical conditions. Those risks include myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the thin sac covering the heart) after mRNA vaccines, and Guillain-Barré syndrome (muscle weakness and changed sensation), and cerebral venous sinus thrombosis (type of blood clot in the brain) after viral vector vaccines.

The study compared the risk of developing a medical condition in various periods after getting a vaccine to the risk before the vaccine became available. The study does not suggest that the vaccines are the cause of the increases, and scientists say more research is needed to determine what causes this increased risk.

Far more Americans were injected with mRNA vaccines, such as the Pfizer and Moderna vaccines, compared to viral vector vaccines, such as the Johnson and Johnson shot. 

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COVID-19 vaccination site

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Activity has increased in the Midwest and Central regions of the country, as well as in the Northeast.


The size of the study, which involved 99 million patients, helped scientists nail down potential risks. 

"Very rare health outcomes, which could be vaccine-associated, may not be seen until hundreds of thousands to millions of individuals have been vaccinated. To identify a two-fold risk of a vaccine-associated adverse event that occurs in once in 100,000 people, 2.35 million people who have received the vaccine need to be compared with 2.35 million people who have not," the Global Vaccine Data Network said. "Ongoing vaccine safety surveillance is essential after vaccines receive emergency-use authorization and/or licensure and vaccines are administered in the general population."

 “The size of the population in this study increased the possibility of identifying rare potential vaccine safety signals,” study lead author Kristýna Faksová of the Department of Epidemiology Research, Statens Serum Institut, said. “Single sites or regions are unlikely to have a large enough population to detect very rare signals.”

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Numerous study participants reported having malaise, fatigue and dizziness six months after their initial COVID infection.


While there are still plenty of lingering questions on COVID-19 shots, researchers say that offering this data provides transparency to the public. The researchers say their research will appear on a public dashboard.

“By making the data dashboards publicly available, we are able to support greater transparency, and stronger communications to the health sector and public," Global Vaccine  Data Network  Co-Director Dr. Helen Petousis-Harris said.

The CDC recommends nearly everyone 6 months and older get an updated COVID-19 vaccine. Updated COVID-19 shots were released last September.