Moderna Inc.’s COVID vaccine generated more than double the antibodies of a similar shot made by Pfizer Inc. and BioNTech SE in research directly comparing immune responses to the inoculations.

A study of almost 2,500 workers at a major Belgium hospital system found antibody levels among individuals who hadn’t been infected with the coronavirus before getting two doses of the Moderna vaccine averaged 2,881 units per milliliter, compared with 1,108 units/mL in an equivalent group who got two jabs of the Pfizer shot.

The results, published on August 31 in a letter to the Journal of the American Medical Association, suggested the differences might be explained by the:

·higher amount of active ingredient in the Moderna vaccine—100 micrograms, versus 30 micrograms in Pfizer-BioNTech

·longer interval between doses of the Moderna vaccine—four weeks, versus three weeks for Pfizer-BioNTech

Moderna’s vaccine was associated with a two-fold risk reduction against breakthrough SARS-CoV-2 infections compared to Pfizer’s in a review of people in the Mayo Clinic Health System in the U.S. from January to July. The results were reported in a separate study released ahead of publication and peer review on Aug. 9.