Table 1.

Proportion of the Treatment Effect Explained by Log₁₀ Hemagglutination Inhibition Titer as the Correlate of Protection Using the Freedman Method

Influenza Illness Endpoint	Proportion of Vaccination Effect
	Observed From Clinical Trial	Estimated From Resampling (Bootstrap) Method
		Mean	Median	2.5th Percentile	97.5th Percentile
RT-PCR–confirmed influenza illness: A/H1N1	0.750	0.792	0.755	0.367	1.479
RT-PCR–confirmed influenza illness: A/H3N2	1.461	1.503	1.462	1.020	2.180

Per-protocol correlate of protection. Hemagglutination inhibition titer was measured at 28 days after last vaccination. Mean, median, 2.5th percentile, and 97.5th percentile of the proportion of the treatment effect were calculated using the bootstrap method with unrestricted random sampling. The parameter calculated can be >100% using this technique, but in this case, it is considered to be 100%.

Abbreviation: RT-PCR, reverse-transcription polymerase chain reaction.