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.

In this study, we observed that seasonal RSV incidence (982-2531 cases per 10 000) was substantially higher than what was reported in each of the prior studies. Some differences may be attributed to the study design and source population. We recruited from a well defined and stable population cohort with complete healthcare utilization data, and we used the same cohort to generate RSV and influenza incidence estimates. Surveillance, active recruitment, and testing occurred during the months when RSV and influenza activity were highest, and we used data from the Wisconsin State Laboratory of Hygiene to estimate the number of additional cases that would have been captured if we had enrolled continuously from week 40 through week 18. This approach created a high level of internal validity. In the NVSN study, results from participating children were applied to national survey data (NAMCS), and the lower RSV incidence may reflect differences in healthcare utilization for ARI at the national level compared with the communities where studies were conducted. The IISP used a convenience sample to obtain viral diagnostic test results, and there was a potential for selection bias with the use of clinical diagnostic testing. It is also unclear whether all respiratory illness visits were captured, because the analysis included only visits to the primary medical provider.