We also computed the population prevented fraction of prematurity for the various periods of influenza activity using the formula:p p c (1{OR) p p c (1{OR)zOR wherep p c is the proportion of cases vaccinated and OR is the odds ratio approximating relative risk (we verified this assumption for our data). The formula, based on the approach described by Kleinbaum et al. [14] , is suitable for computing the population prevented fraction when adjusted measures of association are used. The population prevented fraction for a vaccine estimates the reduction in an outcome given the efficacy/effectiveness of the vaccine and the specific vaccine coverage. We used Stata version 10 (Stata Corporation) for statistical analysis. Identified associations were evaluated for statistical significance at a = 0.05 using two-tailed tests, and Taylor series linearization methods [15] were used to estimate variance.