When the Delta variant of the coronavirus was prevalent in the United States, recipients of two doses of Moderna’s mRNA vaccine were less likely to experience a breakthrough infection – and if they did, were slightly less likely to be hospitalized – than recipients of two doses of the mRNA vaccine from Pfizer and BioNTech, a large study found.

Researchers analysed health records of more than 637,000 vaccine recipients who were not previously infected with the virus and had not gotten a booster shot.

As reported on Thursday in JAMA, breakthrough infections steadily increased every month from July to November 2021, with higher rates in the Pfizer/BioNTech group. In November, there were 2.8 cases among every thousand people vaccinated with the Pfizer/BioNTech shots, compared to 1.6 cases per thousand recipients of the Moderna vaccines. The vaccines protected equally well against death, but the hospitalisation rate was 12.7 per cent for infected Moderna recipients and 13.3 per cent for Pfizer/BioNTech recipients. When the researchers compared 62,584 Moderna recipients to a closely-matched equal-sized group of Pfizer/BioNTech recipients, the risk for breakthrough infection was 15 per cent lower in the Moderna group.

“Although there is a difference in breakthrough infections, both vaccines are highly protective against SARS-COV2 infection and especially against the most severe consequences of infection,” said coauthor Pamela Davis of Case Western Reserve University School of Medicine in a statement.