Antibody titer protection less than believed for H3N2, H1N1

An antibody titer of 1:40 was associated with significantly less than 50% protection against influenza A(H1N1) and influenza A(H3N2) within households, researchers from the University of Hong Kong have found. Previous data suggested this level measured by the hemagglutination inhibition (HAI) assay was associated with 50% protection compared with titers of less than 1:10, the researchers wrote. “An HAI titer of 1:40 was not sufficient to provide 50% protection against PCR-confirmed influenza virus infection in households, potentially because of exposures of greater duration or intensity in that confined setting,” the researchers wrote in The Journal of Infectious Diseases. The researchers identified 297 individuals with PCR-confirmed seasonal influenza A(H1N1) or A(H3N2) for the study, then followed 558 of their household contacts for 7 to 10 days. They collected serum specimens from 198 contacts to measure antibody levels by HAI and microneutralization assays and compared protection among individuals with an HAI titer level of 1:40...