Incidence of Medically Attended Respiratory Syncytial Virus and Influenza Illnesses in Children 6–59 Months Old During Four Seasons

Simpson, Melissa D.; Kieke, Burney A.; Sundaram, Maria E.; McClure, David L.; Meece, Jennifer K.; Sifakis, Frangiscos; Gasser, Robert A.; Belongia, Edward A.

Poisson regression with analytic weights, offsets, and robust variance estimation was used to implement the above extrapolation and standardization procedures for estimating seasonal incidence and 95% confidence intervals (CIs) [17] [18] [19] . The analysis data set comprised enrolled patients. Enrolled patients that did not receive an ICD-9 code for ARI were assigned analytic weights of 1.0. Otherwise, analytic weights equal to the inverse of the proportion of cohort members with MAARI that were enrolled were assigned within strata defined by age group (6-11, 12-23, and 24-59 months) and season. These analytic weights were then multiplied by the appropriate adjustment factors as described above. The outcome in the regression models was the dichotomous virus test result (coded 1 = positive, 0 = negative). The offset terms were stratified by season and age group. Within each stratum, the offset was the natural logarithm of the following quantity: number of cohort members divided by the sum of analytic weights. All calculations were performed using SAS version 9.3 (SAS Institute, Cary, NC).