Table 1. Tools currently in use to evaluate vaccine-induced changes in serum HI titer.

Name	Description	EMA criteria a	FDA criteria b
Mean geometric increase	Ratio of the geometric mean titer (GMT) post vaccination to that pre-vaccination	> 2.5	No standard
Conversion rate or significant increase in titer [n/N (%, 95% CI)] c	Conversion rate: Proportion of subjects showing an increase from a pre-vaccination titer of < 1:10 (non-immune) to a post-vaccination titer of ≥ 1:40 (designated immune state). Significant increase in titer: Proportion of subjects showing a 4-fold or greater increase from a pre-vaccination titer of ≧ 1:10 (designated as a significant increase in titer).	> 40%	LL of 95% CI > 40% d
Protection rate [n/N (%, 95% CI)] c	Proportion of subjects showing a post-vaccination titer of ≧ 1:40	> 70%	LL of 95% CI > 70% e

^a European Medicines Agency (EMA) criteria for serum HI antibody responses in people aged 18–60 y. ^b US Food and Drug Administration (FDA) criteria for serum HI antibody responses in adults < 65 y. ^c n = the number of subjests who meet a requirement; n = total number of subjects; 95% CI = the 95% Confidence Interval. ^d Lower limit (LL) of the 95% CI should exceed 40%. ^e Lower limit (LL) of the 95% CI should exceed 70%.