THURSDAY, Aug. 3, 2023 (HealthDay News) -- While both the Moderna and Pfizer COVID-19 vaccines are considered safe and highly effective, new research finds the Moderna shot has been the safest and most effective for seniors.

“The results of this study can help public health experts weigh which mRNA vaccine might be preferred for older adults and older subgroups, such as those with increased frailty,” said lead study author Daniel Harris. He is an epidemiologist and research scientist in the Center for Gerontology and Healthcare Research at Brown University's School of Public Health, in Providence, R.I.

Researchers compared the two vaccines head-to-head in more than 6 million older adults included in the study.

With an average age of 76, participants were vaccinated against COVID-19 using one of the two vaccines, which have subtle differences in manufacturing, administration and immune response.

The risk of serious adverse events was very low for both vaccine groups. The Moderna vaccine was associated with a 4% lower risk of pulmonary embolism, which is a sudden blockage in blood vessels of the lungs. It was also associated with a 2% lower risk of thromboembolic events (several conditions related to blood clotting).

Taking the Moderna vaccine was also associated with a 15% lower risk of diagnosed COVID-19 compared to the Pfizer vaccine.

This risk of adverse events from an infection with the virus is substantially higher than the risk of adverse events from either vaccine, Harris noted.

But it makes sense to provide detailed information about vaccine effectiveness and safety now that more than 70% of the global population has received one type of COVID-19 vaccine and supply is less of a concern.

“Immunization with either mRNA vaccine is substantially better and safer than not being vaccinated at all,” Harris said. “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.”

Harris also noted the need to understand vaccine performance in real-world populations.

Older adults, who tend to have chronic health conditions, often are excluded from clinical trials. Studying this is especially important given that older adults in nursing homes had a higher risk of developing severe COVID infections.

One reason that Moderna may have improved safety for adverse events like pulmonary embolism is that it offers more protection against COVID-19 for older adults, the researchers said.

“We think that these two things, safety and effectiveness, are interrelated,” Harris said. “The slightly reduced risk of pulmonary embolism and other adverse events that we saw in individuals who received Moderna may be because the Moderna vaccine was also more effective at reducing COVID-19 risk.”

The study could not conclude whether the differences in adverse events were due to safety or effectiveness. The researchers also only looked at the first dose of an mRNA vaccine, so more research is needed.

“You can imagine regularly updating these types of analyses as new vaccines are developed,” Harris said. “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.”

This research was part of a project called the IMPACT Collaboratory, led by researchers at Brown University and Boston-based Hebrew SeniorLife. It is monitoring long-term safety and effectiveness of COVID-19 vaccines for Medicare beneficiaries, in collaboration with CVS and Walgreens pharmacies.

“Because we had these real-world data and a cohort that included millions of older adults, we were able to tease apart potentially very small differences in vaccine safety and effectiveness, and perform analyses on important clinical subgroups,” Harris said.

The research, funded by the U.S. National Institute on Aging, was published online Aug. 2 in JAMA Network Open.

More information

The U.S. Centers for Disease Control and Prevention has more on COVID-19.

SOURCE: Brown University, news release, Aug. 2, 2023