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 spanning 4 seasons, RSV was the most common viral cause of medically attended visits for ARI in children 6-59 months of age. The incidence of RSV was highest among children 6-11 months old, but the 6-11 month and 12-23 month age groups had similar RSV incidence. We were unable to estimate incidence in younger children because they were not eligible for enrollment in the vaccine effectiveness study from which our data were obtained. The incidence among children 6-23 months old was approximately double the incidence in those 24-59 months old. In contrast, the incidence of medically attended influenza was highest among children 24-59 months old. The seasonal incidence of both RSV and influenza was highest during the 2007-2008 season when A/H3N2 was the dominant circulating influenza virus. The overall incidence of influenza was similar in 2007-2008 and during 2009-2010, which included the pandemic. In the latter season, the vast majority of influenza cases occurred in the pandemic wave that affected the community in October and November 2009. The lowest incidence of both viruses occurred in 2006-2007 when seasonal A/H1N1 accounted for the majority of influenza cases. These findings add to the evidence that the burden of RSV is high during the first 24 months of life, and they emphasize the need for a safe and effective intervention to prevent RSV in infants and toddlers. Two other published studies estimated the population incidence of RSV illness in the outpatient setting. The first study was conducted by the New Vaccine Surveillance Network (NVSN) from 2000 to 2004 and estimated the incidence of outpatient RSV illness among children under 5 years old near 3 cities: Nashville, Tennessee, Cincinnati, Ohio, and Rochester, New York [6] . Children with ARI were enrolled and tested for RSV 1-2 days per week from emergency departments, hospitals, and a small number of pediatric practices. Incidence rates were estimated by multiplying RSV-attributable illness among enrolled children by rates of ARI in children from the National Ambulatory Medical Care Survey (NAMCS). Investigators estimated the incidence of RSV to be from 610 to 990 cases per 10 000.