The overall benefits of COVID-19 vaccines far outweigh potential risks, according to a recent study of 99 million vaccinated people – despite a number of misleading and sensational reports about the study. 

The study, published in the journal Vaccine in mid-February, aimed to get more precise estimates of the risk of adverse events following vaccination.

Researchers used data collected from over 99 million vaccinated individuals, who took a total of 183 million Pfizer doses, 36 million Moderna doses and 23 million AstraZeneca doses. 

The study specifically noted a slightly elevated risk of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the lining around the heart) following Pfizer and Moderna mRNA vaccines, and Guillain-Barré syndrome and cerebral venous sinus thrombosis (blood clot in the brain) after taking the AstraZeneca shot. 

“The risk of side effects like myocarditis from the vaccine is extremely low; the risk of developing it from COVID can be many times higher. The study is just reinforcing what we already learnt from previous studies,” stressed Professor Nadav Davidovitch, an epidemiologist, public health physician and head of Ben-Gurion University of the Negev’s School of Public Health. 

Rare adverse events

“Anytime a medicine or a vaccine is approved and introduced into wider use, rare adverse events are found in the population that would be impossible to find in clinical trials,” Prof Katrine Wallace, an epidemiologist at the School of Public Health at the University of Illinois at Chicago, told  Health Policy Watch.

COVID-19 vaccines were globally introduced in 2020, just a year after the virus emerged. The swift rollout prompted intense monitoring, surpassing most other vaccines or drugs. 

“The study methodology examined the observed versus expected ratios: observed post-vaccination rates from a pre-specified list of adverse events to the expected rates, or the historical, background rate of the same events calculated from the same populations during 2015-2019, before the vaccines were available,” Wallace explained in a recent op-ed on MedPage Today. 

This observational study looked at 13 adverse events of particular interest across neurological, haematological and cardiac outcomes and found four statistically relevant ones. 

Guillain-Barré syndrome occurred after the first dose of AstraZeneca’s viral vector vaccine. The observed to expected (OE) ratio was 2.49.

Cerebral venous sinus thrombosis, also known as TTS blood clots, happened after the first AstraZeneca dose, with an OE ratio of 3.23.

Acute disseminated encephalomyelitis occurred after the first dose of Moderna’s mRNA vaccine (OE 3.78). However, this wasn’t found in a more thorough follow-up study by the same group.

Myocarditis and pericarditis happened after Pfizer and Moderna’s mRNA vaccines, as well as AstraZeneca’s viral vector vaccine. (OE of slightly over 1.5).

The data was taken from 10 sites across eight countries and processed by researchers from the Global Vaccine Data Network, and the US Centers for Disease Control and Prevention (CDC) funded the study. 

Wallace told Health Policy Watch that myocarditis and pericarditis were more common in young males aged 15 to 24 after the second vaccine dose. However, any myocarditis or pericarditis that did happen was typically mild and resolved independently without requiring extensive treatment like that needed for COVID-induced cases, she added.

One-in-a-million

Cerebral venous sinus thrombosis was linked to the viral vector vaccines AstraZeneca in this study and Johnson & Johnson in various other studies. Wallace noted that in the United States, approximately six cases were attributed to the J&J vaccine early on. When these cases, mainly affecting older women, were identified, the vaccine was deprioritized, indicating the effectiveness of the monitoring system.

“Many people who don’t understand vaccine safety surveillance think this information was hidden or the trials were not long enough,” Wallace said. “If something is one-in-a-million, you would need a million people in a clinical trial to find one case. We only find rare events after rollout.”

There is no vaccine or medical treatment with no risk, explained both Wallace and Davidovitch.

“The bottom line is that no medication or vaccine is 100% effective or 100% safe,” Wallace said. “Everything has risks and benefits. However, any risks from medications and vaccines must be balanced against the dangers of the disease you are trying to prevent.”

She added that “COVID-19 has killed over a million people in the US and has led to permanent disability in many others. The vaccines are very safe overall, and the benefit of the vaccine still definitely outweighs the risks.”

Booster Vaccines for Seniors

On Wednesday, the CDC recommended that adults over 65 receive an updated COVID-19 vaccine shot after previously recommending boosters for people who are immunocompromised. 

The CDC statement highlighted that individuals aged 65 and older are particularly affected by COVID-19. It reported that over half of the COVID-19 hospitalizations from October 2023 to December 2023 involved people in this age bracket.

The CDC “recommendation allows older adults to receive an additional dose of this season’s COVID-19 vaccine to provide added protection,” said CDC Director Mandy Cohen. “Most COVID-19 deaths and hospitalizations last year were among people 65 years and older. An additional vaccine dose can provide added protection that may have decreased over time for those at highest risk.”

“We are now dealing with the JN.1 COVID strain, which is very infectious,” Davidovitch said. “We still need to be cautious.”

Image Credits: Ron Cogswell.