A booster dose of mRNA vaccine significantly decreases the risk of being infected with either the delta or omicron COVID-19 variants, according to scientists at the Rocky Mountain Center for Occupational and Environmental Health (RMCOEH), a program partnership between the University of Utah and Weber State University.

The researchers found that a booster shot, or third dose, was more than 90% effective against the delta variant and 60% against omicron. In comparison, a two-dose regimen was only 65% effective against delta and 46% against omicron.

Their study appears in the New England Journal of Medicine.

We are thrilled to share the results of this study, which provides valuable information about how the evolution of the coronavirus is affecting one of the key tools in the fight against COVID-19.”

Sarang K. Yoon, DO, MOH, assistant professor at RMCOEH and study’s lead author

The study evaluated COVID-19 infections between late August, 2021 and January, 2022 among 3,975 health care personnel, first responders and other frontline workers in six states who had received a booster shot of an mRNA vaccine. The workers had either received three doses from a single manufacturer — Moderna or Pfizer-BioNTech — or a combination of the two vaccines.

After 17 weeks, the researchers found that among health care workers with SARS-CoV-2 infection, the mean viral RNA load was 40% lower in partially or fully vaccinated participants than in unvaccinated participants. In addition, the risk of fever was 58% lower, and the duration of illness was shorter, with 2.3 fewer days spent sick in bed.

Based on these findings, the researchers concluded that mRNA vaccines continue to provide robust protection against severe outcomes.

“These results show that in this real-world study of workers, the vaccine continues to be effective against the omicron variant of the virus, although at a reduced rate of efficacy compared with the prior delta variant,” Yoon said.

Overall, these results confirm that mRNA vaccines are not only highly effective in preventing SARS-CoV-2 infection, but also may mitigate the effects of breakthrough infections. This finding is particularly important to essential and frontline workers, given their potential to transmit the virus through frequent close contact with patients, coworkers, and the public, the researchers conclude.

“Researchers across the globe have stepped up over the last two years to address this crisis,” Yoon said. “We are pleased to have played a role in helping the world understand the disease and the vaccines’ ability to protect against it.”

Future studies will examine effectiveness of a second booster shot, or fourth dose of mRNA vaccine.