Immunity against the coronavirus is waning in people who were fully vaccinated with the shot made by BioNTech SE and Pfizer Inc. in January because of the rapidly spreading Delta variant, BioNTech’s chief executive said, confirming data that emerged from Israel last week.

But even as antibody levels are dropping seven months after immunization among some vaccine recipients, most of them will remain protected against severe disease and might not yet need a third dose, according to Ugur Sahin, CEO of the German company that invented the vaccine and partnered with Pfizer to develop the product for the global market.

“The antibody titers are going down,” Dr. Sahin said, referring to the unit of measurement for antibodies against the virus. “The vaccine protection against the new variant is considerably lower.”