Study Suggests COVID-19 Vaccine May Increase Risk of Myocarditis, Guillain-Barré Syndrome, Other Side Effects

The risks associated with developing severe illness from COVID-19 are still more than 600 times higher than the potential health side effects from the vaccine, which have likely saved millions of lives, researchers noted.

COVID-19 vaccines have been widely administered and proven effective at helping to protect against serious infection and death over the past few years. However, the findings of a new study suggest that side effects may pose a small increased risk of neurological, blood, and heart conditions for some individuals.

In findings published last week in the medical journal Vaccine, researchers examined data from nearly 100 million participants, confirming that the COVID-19 side effects may include an increased risk of myocarditis, pericarditis, Guillain-Barre syndrome, the neurological disorder acute disseminated encephalomyelitis, and other potential health complications.

An international team of researchers used data from the Global COVID Vaccine Safety Project (GCoVS), established in 2021 under the multinational Global Vaccine Data Network, to evaluate the potential side effects of COVID-19 vaccines. The study included participants at 10 sites across eight countries, who received Pfizer, Moderna, and AstraZeneca COVID-19 vaccines, and focused on 13 side effects across neurological, blood, and heart conditions.

Researchers looked for side effects occurring up to 42 days after vaccination with Pfizer and Moderna mRNA and AstraZeneca viral-vector vaccines. It is the largest COVID vaccine study conducted to date, and included data from 183 million doses of the Pfizer mRNA vaccine, 36 million doses of the Moderna mRNA vaccine, and 23 million doses of the AstraZeneca adenovirus-vector vaccine.

According to the findings, the risk of pericarditis was nearly seven times higher among those who took a third dose of AstraZeneca’s viral vector vaccine. It was 1.7 and 2.6 times higher among patients who took a first and fourth dose of Moderna’s mRNA vaccine, respectively.

The data indicates patients who received the first, second, and third doses of any mRNA vaccines faced a slightly increased risk of myocarditis, which is a heart condition marked by inflammation of the outer lining of the heart. The risk was six times higher after the second Moderna dose.

The COVID-19 vaccines were also linked to a potential risk of pericarditis, which involves swelling of the saclike tissue that surrounds the heart. Both myocarditis and pericarditis can lead to symptoms like a heart attack, including sharp chest pains.

Guillain-Barre syndrome is a condition that affects the nerves, which can cause weakness and tingling in the feet and legs that can spread to other parts of the body. Researchers found that the risk for Guillain-Barre syndrome from the COVID-19 vaccine was 2.5 times higher, and individuals who received the AstraZeneca vaccine were three times more likely to develop venous sinus thrombosis, which is a condition that causes blood clots in the major veins in the brain and can cause headaches, seizures, and weakened limbs.

The researchers also determined patients faced nearly four times the risk of developing acute disseminated encephalomyelitis, a type of inflammation and swelling of the brain, when they took the Moderna vaccine. They also faced twice the risk of the condition if they took the AstraZeneca vaccine.

COVID-19 Risks Far More Dangerous

Researchers said the data confirmed a small risk of developing the various conditions based on the type of vaccine that was administered. But the risk of contracting COVID-19 and facing serious infection, complications, or death is still much higher than the risk from the blood, brain, and heart conditions outlined in the study.

The risk of having a neurological side effect after COVID-19 infection is 617 times higher than the risk faced after receiving any COVID-19 vaccine. The same is true for developing myocarditis or another heart condition after contracting the COVID virus compared to the COVID vaccine, they determined.

So far, roughly 71% of the world’s population has received at least one dose of a COVID vaccine. More than 13.5 billion doses have been administered since the start of the pandemic.

Despite an uptick in COVID-19 infections this winter from the JN.1 variant, COVID cases, hospitalizations, and deaths are declining overall, researchers say, due to the widespread administration of COVID-19 vaccines.