This study states that Viral detection is the gold standard for measuring incident influenza infections.1, 2 Yet, high asymptomatic infection rates and transient viral presence during infections3, 4 limit detection when using virologic outcomes to measure population-based influenza burden. Fourfold antibody titer increases over time (ie, seroconversion) and is traditionally used to measure influenza incidence.1, 2, 5 However, an individual’s immunological response to influenza combines previous and recent influenza exposures.6-9 Current seroconversion methods often do not account for these effects.

Longitudinal sera sampling capture different antibody titer snapshots. Figure 1 illustrates antibody variations before and after infection. Figure 1A shows hypothetical log hemagglutinin inhibition (HI) titers to three influenza strains, sampled antibody response sets over 2 years, and corresponding observed antibody titer changes . Ideally, initial sampling captures sera before infection (ie, baseline sera) (Our results highlight the need for improved methods when using antibody titers as an endpoint in settings where there is no clear influenza “off” season. Methods, like those presented here, that use titers from circulating and non-circulating strains may be key.

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