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(NEXSTAR) – A head-to-head study of the Moderna and Pfizer vaccines has determined the former to be slightly more effective against COVID-19, at least as far as the alpha and delta variants are concerned.

The results of the study, conducted by researchers with Harvard Medical School, the Harvard T.H. Chan School of Public Health, Brigham and Women’s Hospital and the Veterans Administration, were published in the New England Journal of Medicine this week.

Findings indicated that subjects who received the Moderna vaccine (mRNA-1273) were less likely to experience COVID-19 outcomes than those who received the Pfizer–BioNTech vaccine (BNT162b2) during periods of alpha- or delta-variant dominance. Specifically, when the alpha variant was predominant, researchers recorded 4.52 infections per every 1,000 study participants who received the Moderna vaccine, but 5.75 infections per 1,000 among those who received the Pfizer vaccine. During delta predominance, an additional 6.54 infections per 1,000 were recorded in the Pfizer group.

Moderna was also found to be slightly more effective against every category of COVID-19 outcome, including infection, symptomatic infection, hospitalization, ICU treatment or death.

Both of the vaccines, however, are highly effective against COVID-19 outcomes, the study’s authors said.

“Given the high effectiveness of both vaccines, either one is strongly recommended to any individual offered the choice between the two,” said lead author Barbra Dickerman, an instructor of epidemiology and investigator at the Harvard T.H. Chan School’s CAUSALab.

Dickerman acknowledged that while the differences in effectiveness were slight, “they may be meaningful for larger decision-making bodies, such as health care systems and higher-level organizations, when considering the large population scale at which these vaccines are deployed.”

The study, meanwhile, was conducted prior to the outbreak of the omicron variant, meaning more research is needed to assess either vaccine’s effectiveness against the latest mutations.