Over the past several days, the safety and efficacy of messenger RNA (mRNA) vaccines have been under intense scrutiny, fueled by recent regulatory actions. However, infectious disease experts maintain that the technology is safe, effective, and has been instrumental in saving millions of lives.
On Tuesday, the U.S. Food and Drug Administration (FDA) announced plans to limit access to future COVID-19 shots—two of which are mRNA vaccines—to those aged 65 and older or with high-risk conditions. The agency will require further scientific trials to greenlight the shots for younger age groups.
The agency also sent letters to both Moderna and Pfizer last month, telling them to expand the warning labels on their mRNA COVID-19 vaccines to broaden the people who may be impacted by the risk of heart inflammation as a possible side effect.
Despite the recent headlines, infectious disease experts told ABC News that mRNA and its use in vaccines have been studied for decades.
“Here’s the bottom line: mRNA vaccines for COVID, according to estimates from Yale School of Public Health, saved 3.2 million lives,” Dr. Peter Hotez, a professor of pediatrics and molecular virology at Baylor College of Medicine, told ABC News.
“So instead of 1.2 million Americans who lost their life because of COVID, it would have been 4.4 million,” he added. “So, I think it's unfortunate that anti-vaccine activists target mRNA vaccines like they do, but it is a good technology.”
mRNA was discovered independently by two teams in 1961. Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco, noted that breakthroughs in developing mRNA vaccines began in the early 2000s, paving the way for the COVID-19 vaccines in 2020.
While most vaccines use a weakened or inactivated virus, mRNA vaccines work differently by teaching the body how to make proteins that trigger an immune response.
“The way that it works is that it doesn't even go into the nucleus [of the cell]. It enters the outside of the cytoplasm... and basically instructs the cell to make proteins,” Chin-Hong told ABC News. “But most importantly, it self-destructs in a matter of, at the most, days, and it dies.”
He continued, “So mRNA goes away, but the products which are the most important thing -- the proteins and antibodies -- remain, and that's why we get protection.”
Chin-Hong also addressed misinformation suggesting mRNA vaccines could alter DNA: “Our cells can't convert mRNA to DNA because the mRNA doesn't enter the DNA, which is in the nucleus.”
Evidence for the safety of mRNA vaccines comes from multiple sources:
Large-Scale Clinical Trials: In 2020, the initial Pfizer-BioNTech and Moderna trials involved a combined total of over 70,000 people. Furthermore, Moderna’s clinical trials for its RSV vaccine involved 37,000 people.
Known Side Effects: Researchers found that common side effects (fever, arm pain) were similar to those of traditional, non-RNA vaccines and that the shots had short-term efficacy rates of over 90%.
Ongoing Safety Monitoring: Post-launch studies have consistently found that booster safety aligns with the primary vaccination series.
“There are all these databases that are used to follow reports of people... There have been multiple studies since 2020 showing there's no impact in fertility, stroke, all the things that people have worried about,” said Chin-Hong.
Dr. Hotez acknowledged that no vaccine technology is perfect but highlighted the advantages of mRNA, such as the ability to design and deploy them more quickly than traditional vaccines. He disagreed with the FDA's decision to limit future shots, citing the long-term risks of COVID-19 itself.
“I think there are many younger adults... who are concerned enough about long COVID or downstream heart disease to want to be able to get the mRNA vaccine,” he said.
Myocarditis, an inflammation of the heart muscle, has been a central point of concern. The condition can cause arrhythmias (abnormal heartbeats) and weaken the heart muscle.
According to the CDC, cases of myocarditis and pericarditis (inflammation of the sac around the heart) have been observed rarely after COVID vaccination. When these rare cases have occurred, they have been most common among young adult males (ages 18-29) within a week of receiving their second mRNA dose.
The FDA's request to expand warning labels was based on new data from its safety surveillance systems.
However, experts emphasize that the context is critical. Dr. Chin-Hong stated that the risk of developing myocarditis is much higher from a COVID-19 infection than from the vaccine.
“The risk of COVID is much higher in general. If you look at it, [myocarditis risk from the vaccine is] 22 to 31 cases per million [among] 18 to 29 years old as an example,” he said. “At the time when these vaccines are used very often in that group, [myocarditis risk from COVID-19 infection] is 1,500 per million. So, you're talking about 22 to 31 per million versus 1,500 per million.”
NOTE: Reprinted from ABC News. Youri Benadjaoud contributed to this report.
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