Clinical characteristics of influenza A/H1N1, A/H3N2 and B infections in children during three consecutive winter seasons (2012-13, 2013-14 and 2014-15)

BACKGROUND: Influenza in children ranges from subclinical to complicated disease, however few data are available regarding the possibility of differences in clinical manifestations following infection with different influenza viruses. We report the results of a retrospective study regarding clinical symptoms of laboratory-confirmed influenza A/H1N1, A/H3N2 or B infection in subjects aged less than 15 years in three consecutive winter seasons (2012-13, 2013-14 and 2014-15). METHODS: Real-time reverse polymerase chain reaction was performed in throat/nasal swabs collected from children presenting influenza like illness to identify A and B influenza viruses and to subtype samples positive for influenza A. Demographic characteristics, underlying medical conditions, clinical symptoms and influenza vaccination were collected for each patient. RESULTS: A total of 359 swabs was collected: 348 by pediatricians of the Italian Influenza Surveillance Network (InfluNet) and 11 by hospital staff. Influenza viruses were detected in 203 samples: 141 were influenza A (67 A/H1N1 and 74 A/H3N2) and 63 influenza B. One patient was positive for influenza A/H3N2 and B. The mean age of children was 4.2 ± 3.0 for A/H1N1, 5.7 ± 3.4 for A/H3N2, and 5.6 ± 2.5 for B. The mean age of the 10 children hospitalized with laboratory confirmed influenza (3.3± 3.0) was lower as compared with patients not requiring hospitalization (p<0.05) and frequently caused by A/H1N1viruses (6/10) The most common symptoms observed were fever (100%), rhinorrhea (94%), cough (92%), and sore throat (65%). Comparing the symptoms found in children positive for influenza A or B, the respiratory ones were similar, whereas headache and gastrointestinal symptoms were associated more frequently with influenza B infection (p<0.05). Comparing children infected with A/H1N1 or A/H3N2 viruses, rhinorrhea (p<0.01), headache (p<0.01) and sore throat were found to be more frequent in A/H3N2 infected children. CONCLUSION: Although the number of observations was limited, our results seem to suggest a higher frequency of some symptoms in children infected with influenza B or A/H3N2 viruses. Influenza A/H1N1 infections were found to be more frequent among patients in the first five years of life.