Comparing Safety, Efficacy of COVID-19 mRNA Vaccines in Older Adults


Moderna’s mRNA-1273 vaccine was associated with a lower risk of diagnosed COVID-19 when compared to Pfizer’s BNT162b2.

Moderna’s COVID-19 mRNA vaccine was associated with a lower risk of pulmonary embolism and other adverse events in older adults when compared to Pfizer’s, according to recent data published in JAMA Network Open.1

Although the mRNA vaccines from both pharmaceutical companies have proven to be safe and effective, there is a lack of data about potential differences in older adults. There also has been few studies which compared both vaccines, but some have shown that meaningful differences in the risk of adverse events that can vary by age may exist.

The study, conducted by a team of researchers from Brown University, aimed to address the gap in data between the general population and older adults.

“Immunization with either mRNA vaccine is substantially better and safer than not being vaccinated at all,” Daniel Harris, an epidemiologist and research scientist the Brown University School of Public Health, said in a release.2 “But in an ideal world where we can have a choice between which vaccine product is used, we wanted to see whether one vaccine was associated with better performance for older adults and those with increased frailty.”

Investigators conducted a retrospective cohort study to compare the risk of adverse events between Moderna and Pfizer’s mRNA vaccines for COVID-19 overall, by frailty level, and by prior history of certain adverse events. Data was gathered from a database of community pharmacy and Medicare claims.

The study cohort included 6388196 participants who received either Moderna’s mRNA-1273 vaccine or Pfizer’s BNT162b2 vaccine. Of those, 59.4% were women and 86.5% were white with a mean age of 76.3 years. A total of 38.1% of participants were categorized as prefrail and 6% as frail.

Primary study outcomes were 12 serious adverse events, including acute myocardial infarction, facial nerve palsy, deep vein thrombosis, Guillain-Barre syndrome, disseminated intravascular coagulation, hemorrhagic stroke, and myocarditis or pericarditis. The risk of diagnosed COVID-19 was assessed as a secondary outcome.

Investigators found that the risk for adverse events was low in both vaccine groups. However, Moderna’s vaccine was associated with a lower risk of pulmonary embolism and other adverse events, such as thrombocytopenia purpura. Additionally, Moderna’s vaccine was associated with a lower risk of diagnosed COVID-19, which was attenuated by frailty level.

Study limitations include rare outcomes that are hard to examine with precision, residual confounding, nonrandom selection or administration due to early perceptions in differences of vaccine performance, a possibility of incomplete outcome ascertainment, and challenges in determining the timing of adverse events.

“You can imagine regularly updating these types of analyses as new vaccines are developed,” said Harris. “Depending on which one comes out on top, even on a very small scale, that may have big implications at the population level and render a preference for that particular vaccine.”

1. Harris DA, Hayes KN, Zullo AR, et al. Comparative Risks of Potential Adverse Events Following COVID-19 mRNA Vaccination Among Older US Adults. JAMA Netw Open. 2023;6(8):e2326852. doi:10.1001/jamanetworkopen.2023.26852
2. Moderna is safest, most effective mRNA vaccine against COVID-19 for older adults, study shows. News Release. Brown University. August 2, 2023. Accessed August 2, 2023.
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