Intervention rate ratios for influenza infection.

aIntracluster correlation coefficient: −0.0014 in unadjusted model (N = 1,111), −0.0030 in model adjusting for gender, race, ethnicity, smoking status, physical activity, and having ever received a vaccination for influenza (N = 986). bcRR = Cumulative Rate Ratio is the week by treatment effect which is equivalent to the hazard ratio over the study period. c95% CI, 95% confidence interval. Significance level set at P<0.05.