But by now, the observational studies have delivered results from a number of locations — Qatar, the Mayo Clinic in Minnesota, several other states in the United States — and in health care workers, hospitalized veterans or the general population.
Moderna’s efficacy against severe illness in those studies ranged from 92 to 100 percent. Pfizer-BioNTech’s numbers trailed by 10 to 15 percentage points.
The two vaccines have diverged more sharply in their efficacy against infection. Protection from both waned over time, particularly after the arrival of the Delta variant, but the Pfizer-BioNTech vaccine’s values fell lower. In two of the recent studies, the Moderna vaccine did better at preventing illness by more than 30 percentage points.
A few studies found that the levels of antibodies produced by the Pfizer-BioNTech vaccine were one-third to one-half those produced by the Moderna vaccine. Yet that decrease is trivial, Dr. Moore said: For comparison, there is a more than 100-fold difference in the antibody levels among healthy individuals.