Pfizer Inc. and BioNTech SE said that a third dose of their Covid-19 vaccine neutralized the Omicron variant in lab tests but that the two-dose regimen was significantly less effective at blocking the virus.

A third dose increased antibodies 25-fold compared with two doses against the Omicron variant, the companies said. Still, two doses may prove effective in preventing severe illness from Covid-19, they said, because immune cells are able to recognize 80% of parts of the spike protein that the vaccine targets.