Figure 3.

Probability of protection against reverse-transcription polymerase chain reaction–confirmed influenza illness according to log₁₀ hemagglutination inhibition (HI) titer predicted by the Dunning model (per-protocol correlate of protection; HI titer measured at 28 days after last vaccination). The density of log₁₀ HI antibody titer lines show the distribution of postvaccination HI titers in study participants (the proportion of children with titers of the shown level). The graphs begin at a log₁₀ HI antibody titer of 0.7, the assay cutoff level. As expected, a few children in the inactivated quadrivalent influenza vaccine (IIV4) group had very low or very high titers, but most had titers in the mid-range. In contrast, most children in the control group had low HI antibody titers.