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.

Residual swab samples were tested for 2483 study enrollments 6-59 months old from the 2006-2007 through the 2009-2010 seasons. Of these, 2384 (96.0%) yielded conclusive results and 99.6% occurred in the outpatient setting. Real-time PCR was positive for RSV and influenza in 627 (26.3%) and 314 (13.2%) cases, respectively. The mean age (standard deviation) for RSV-positive children was 28 (15) months and 38 (16) months for influenza-positive children. Seventy-seven (24.5%) influenza cases received 2 doses of influenza vaccine at least 14 days prior to symptom onset. Respiratory syncytial virus was the most frequent single virus identified, but other respiratory viruses were commonly detected. Influenza, human metapneumovirus, human rhinovirus, coronavirus, and parainfluenza (types 1-4) were each detected as virus infections in 8% or more of children (5% or more as single infections) ( Table 1) . Twenty-one percent of samples were negative for all viral targets. Respiratory syncytial virus was the most common coinfecting agent, with RSV and noninfluenza viruses comprising the largest group of coinfections (6.3%). Respiratory syncytial virus A accounted for 61% of all RSV infections. Type A accounted for 59% to 64% of RSV infections across different age groups. The proportion due to RSV A varied by season, ranging from 32% in 2008-2009 to 87% in 2006-2007.