Quantifying Protection Against Influenza Virus Infection Measured by Hemagglutination-inhibition Assays in Vaccine Trials.

Long-term immunity against influenza virus infection is thought to be primarily conferred by the adaptive immune system, and specifically antibody-mediated humoral immunity.1 Important historical studies identified a correlation between the antibody titer measured by the hemagglutination-inhibition assay with protection against influenza virus infection,1 suggesting that an hemagglutination-inhibition titer (hereafter referred to as “titer”) of 40 or higher could provide ~50% protection against infection.2 Subsequent studies support this finding.3 Titers are rapidly boosted by infection or vaccination peaking within 2–4 weeks of either event,4 and then wane over time.5 This leads to fluctuations in the level of herd immunity that may drive the timing and magnitude of epidemics.6 Knowing the protection conferred by antibodies corresponding to a given titer may shed light on the seasonality of influenza in tropical/subtropical regions, where outbreaks may occur year round or several times a year, in contrast to temperate regions.7,8 Identifying the degree of correlation of titers with protection against infection is also important for vaccine development, because annual reformulations of inactivated influenza vaccines can now be approved based entirely on immunogenicity data.9 The dynamism of titers complicates understanding of their protective effect, for they may change substantially over the course of a single study, in which case the titer preseason, or postvaccination, may not adequately characterize the antibody levels at the time of exposure to infection. Furthermore, if entry to the study is staggered, individuals will experience neither the same total quantum nor the same timeline of exposures. Such heterogeneities in exposure add complexity to statistical analysis, for instance, by violating the assumption of proportional hazards in Cox models. Here, we developed a statistical methodology allowing staggered entry and used this to investigate how fast antibody titers waned over time, and how adjustment for antibody waning affected the correlation between titers and protection against confirmed influenza virus infection, in vaccine trials in Hong Kong and the United States.