Immunity to Influenza B/Yamagata‐Lineage Viruses Has Not Waned Since the Disappearance of This Virus Lineage

Background Influenza B viruses are important contributors to seasonal influenza epidemics. Two antigenically distinct lineages, B/Victoria and B/Yamagata, have been circulating since the late 1980s. However, the B/Yamagata lineage has not been detected in most countries since 2020, potentially resulting in waning immunity that may leave people vulnerable to B/Yamagata virus infections, should this lineage reemerge. Methods We investigated the impact of the recent lack of B/Yamagata virus circulation on immunity in adults by analyzing the hemagglutination‐inhibition (HI) titers of serum samples ( n = 504) collected from 2013 through 2024 against influenza B viruses circulating from 1958 to 2019. Results Human serum HI titers to B/Yamagata viruses have not markedly declined since B/Yamagata viruses were last detected in the US in 2020. Human serum HI titers were highest against the first encountered B/Victoria‐ and B/Yamagata–lineage viruses, respectively, revealing imprinting effects. Conclusions The recent disappearance of the B/Yamagata lineage has not led to a substantial decline in antibody levels against this lineage.