There was no statistical interaction by specific influenza season (i.e., 2004-2005 and 2005-2006) for all analyses of prematurity, SGA, and birth weight (for all interaction terms: p.0.05-detailed data available on request). Moreover, the association between maternal influenza vaccine and birth outcomes was qualitatively similar for the two influenza seasons. For example, during the period of widespread influenza activity, the adjusted ORs for prematurity were 0.17 (95% CI, 0.03-0.86) for the 2004-2005 season and 0.32 (95% CI, 0.10-1.14) for the 2005-2006 season.