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 and a reference group of people with titer levels of less than 1:10.
HAI titers of 1:40 was associated with a 31% (95% CI, 13-46) protection against A(H1N1) and a 31% (95% CI, 1-53) protection against A(H3N2). A microneutralization titer of 1:40 was associated with 49% (95% CI, 7-81) protection against A(H3N2). The researchers also found that contacts younger than 18 years were more susceptible to influenza compared with contacts aged 19 to 50 years for both A(H1N1) and A(H3N2). Contacts older than 50 years were less susceptible to A(H1N1) but not A(H3N2) vs. those aged 19 to 50 years.
“Further work could examine how information on HAI titers and [microneutralization] titers could be combined to provide a better immune correlate than the HAI or [microneutralization] titer alone,” the researchers wrote.
Disclosure: The researchers report financial relationships with CLSA, Crucell NV, GlaxoSmithKline, Hoffmann-La Roche, HSBC, MedImmune Inc., Sanofi and Sanofi Pasteur.