Figure 4.

Geometric mean (GMT) neuraminidase inhibition (NAI) titers to influenza A (H3N2) at prevaccination, and 1, 6, 12, and 18 month postvaccination time points, by intervention and 2-year vaccination status*, with estimated regression lines**. *Participation in the 2004–2005 study year was used as a proxy for history of influenza vaccination in that season. Those enrolled in both 2004–2005 and 2005–2006 received the same intervention both years; however, history of vaccination in the 2004–2005 season was not explicitly determined for those newly enrolled in 2005–2006. **Rates of antibody change were estimated in linear mixed models with log₂ titers as the dependent variable and time in days from the 1 month postvaccination blood draw as the independent variable. To account for correlation of titers within individuals over time, the intercept and time were modeled as random effects. Abbreviations: IIV, inactivated influenza vaccine; LAIV, live attenuated influenza vaccine.