Hemagglutination Inhibition Antibody Titers as a Correlate of Protection for Inactivated Influenza Vaccines in Children

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Abstract
The hemagglutination inhibition (HI) titer of 1:40, which has been recognized as an immunologic correlate corresponding to a 50% reduction in the risk of contracting influenza, is based on studies in adults. Neither seasonal nor challenge-based correlates have been evaluated in children. A total of 4707 influenza vaccine-naive healthy children 6 to 72 months old were randomized in a ratio of 2:2:1 to receive 2 doses of MF-59-adjuvanted influenza vaccine (Novartis Vaccines), trivalent inactivated influenza vaccine subunit (trivalent inactivated influenza vaccine control, GSK), or a saline placebo during the 2007 to 2008 and 2008 to 2009 influenza seasons. The second dose was given 30 days after dose 1. Clinical influenza-like illnesses cases identified by active surveillance were confirmed by reverse transcription polymerase chain reaction testing for influenza. Vaccine immunogenicity 50 days after dose 1 was evaluated in a subset of 777 children. Immunogenicity and efficacy results for H3N2 were evaluated against the Prentice criteria, which confirmed that the immunogenicity results warranted estimation of an immunologic correlate. We then used the Dunning model fitting the H3N2 antibody titers at day 50 and the influenza cases observed in the immunogenicity subset to estimate a correlate of protection. This analysis revealed that a cutoff HI titer of 1:110 was associated with the conventional 50% clinical protection rate against infection during the entire season, and titers of 1:215, 1:330, and 1:629 predicated protection rates of 70%, 80%, and 90%, respectively. The conventional adult HI titer of 1:40 was only associated with 22% protection. The use of the 1:40 HI adult correlate of protection is not appropriate when evaluating influenza vaccines in children. Although a cutoff of 1:110 may be used to predict the conventional 50% clinical protection rate, a titer of 1:330 would predict an 80% protective level, which would seem to be more desirable from a public health perspective.