The bivalent booster injections from Moderna Inc. and Pfizer Inc. failed to raise levels of protective proteins called neutralizing antibodies against dominant omicron strains more than four doses of the original Covid vaccine, according to an early independent study in a small group of people.

Researchers from Columbia University and the University of Michigan compared the levels of neutralizing antibodies in blood samples from 21 people who received a fourth injection of the Moderna or Pfizer-BioNTech SE bivalent boosters with antibody levels in 19 people who received four injections of the original vaccines.