Association Between Hemagglutination Inhibition Antibody Titers and Protection Against Reverse-Transcription Polymerase Chain Reaction-Confirmed Influenza Illness in Children 6-35 Months of Age: Statistical Evaluation of a Correlate of Protection

Abstract

Background: Data from a randomized controlled efficacy trial of an inactivated quadrivalent influenza vaccine in children 6-35 months of age were used to determine whether hemagglutination inhibition (HI) antibody titer against A/H1N1 and A/H3N2 is a statistical correlate of protection (CoP) for the risk of reverse-transcription polymerase chain reaction (RT-PCR)-confirmed influenza associated with the corresponding strain.

Methods: The Prentice criteria were used to statistically validate strain-specific HI antibody titer as a CoP. The probability of protection was identified using the Dunning model corresponding to a prespecified probability of protection at an individual level. The group-level protective threshold was identified using the Siber approach, leading to unbiased predicted vaccine efficacy (VE). A case-cohort subsample was used for this exploratory analysis.

Results: Prentice criteria confirmed that HI titer is a statistical CoP for RT-PCR-confirmed influenza. The Dunning model predicted a probability of protection of 49.7% against A/H1N1 influenza and 54.7% against A/H3N2 influenza at an HI antibody titer of 1:40 for the corresponding strain. Higher titers of 1:320 were associated with >80% probability of protection. The Siber method predicted VE of 61.0% at a threshold of 1:80 for A/H1N1 and 46.6% at 1:113 for A/H3N2.

Conclusions: The study validated HI antibody titer as a statistical CoP, by demonstrating that HI titer is correlated with clinical protection against RT-PCR-confirmed influenza associated with the corresponding influenza strain and is predictive of VE in children 6-35 months of age.

Clinical trials registration: NCT01439360.

Keywords: HI antibodies; children; correlate of protection; influenza.

Figure 1.

Hemagglutination inhibition antibody titers in children with or without reverse-transcription polymerase chain reaction–confirmed influenza illness at 28 days after final vaccination (per-protocol correlate of protection cohort). Abbreviations: CI, confidence interval; GMT, geometric mean titer; IIV4, inactivated quadrivalent influenza vaccine; RT-PCR, reverse-transcription polymerase chain reaction.

Figure 2.

Reverse cumulative distribution curves for hemagglutination inhibition (HI) antibody titers in children with or without reverse-transcription polymerase chain reaction (RT-PCR)–confirmed influenza illness at 28 days after final vaccination (per-protocol correlate of protection cohort). The green line shows postvaccination HI titers in inactivated quadrivalent influenza vaccine (IIV4)–vaccinated children who did not experience RT-PCR–confirmed influenza illness. The red line shows postvaccination HI titers in IIV4-vaccinated children with RT-PCR–confirmed influenza illness. The bright pink line shows postvaccination HI titers in unvaccinated (control) children who did not experience RT-PCR–confirmed influenza illness. The pale pink line shows postvaccination HI titers in unvaccinated (control) children with RT-PCR–confirmed influenza illness. The other lines show prevaccination HI titers. The dotted lines show the assay cutoff level (titer ≥10 1/dilution [DIL]) and the level defined in the study protocol as seroprotective (titer ≥40 1/DIL).

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.