Figure 5.

Hemagglutination inhibition (HAI) and neuraminidase inhibition (NAI) titers. HAI and NAI titers were evaluated pre- and postchallenge and in follow-up. A, There was an overall increase in HAI titers among all participants by week 8 compared to day 0 (prechallenge). B, NAI titers were elevated even prior to challenge and remained elevated throughout the study period. C, Participants were grouped by response after challenge. Those participants with a response after challenge had a ≥4-fold increase in HAI titer after challenge and showed a significant rise by week 8. D, Many participants did not have a rise in HAI titer after challenge, and HAI titers remained low throughout the study period. Dotted lines indicate the lowest level of detection. Dashed lines indicate the level of protection (≥1:40). Lines represent geometric mean titers and error bars represent 95% confidence intervals. Abbreviations: HAI, hemagglutination inhibition; NAI, neuraminidase inhibition.