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.

Our study had several limitations. We included outpatient medically attended infections, and children with asymptomatic RSV or mild, nonmedically attended illness were not included. Therefore, the incidence rates underestimate the full burden of RSV in the community. The length of the enrollment period was relatively short-only 10-12 weeks during each season except the 2009-2010 season, which had an extended enrollment period due to the pandemic. We addressed this by using state surveillance data to estimate the proportion of cases occurring outside our enrollment window, but we do not know whether state surveillance data are a valid measure of RSV activity in the community outside the enrollment period. We did not enroll children <6 months of age when serious RSV infections may occur. As a result, we were unable to estimate RSV and influenza incidence in this age group. Rhinoviruses along with RSV and influenza were the majority of viruses detected. However, statewide data combines enterovirus and rhinovirus groups due to genetic similarity. Therefore, we were unable to estimate the proportion of rhinovirus cases occurring outside our enrollment period and did not estimate the incidence of medically attended rhinovirus infections. Finally, we did not detect human bocavirus because the multiplex PCR array did not include it. This may have contributed to the relatively small proportion of children who were coinfected with multiple viruses [22] .