Incidence of influenza and associated illness in children aged 0-19 years: a systematic review

A systematic review was conducted to estimate the incidence of influenza and concomitant morbidity and mortality in children from 0 to 19 years (0-19 years). Medline was searched for observational studies and placebo or non-treated arms of experimental studies providing occurrence rates of laboratory-proven influenza illness. From the 2758 titles identified, 356 full papers were reviewed based on the abstract or title; after searching their reference lists an additional 16 papers were found. Finally 28 studies met our inclusion criteria, reporting a varying seasonal incidence of influenza of up to 46%. However, when analysing two long-term observational studies and averaging seasonal fluctuations, the overall incidence of influenza was found to range from 5% to 9.5% per year. Serious morbidity was seldom reported and no cases of mortality were found. Our review shows influenza as an infection with a moderate average incidence and a self-limiting character that is associated with mild morbidity and rare cases of mortality in children. This systematic review may be subject to two contrasting biases. First, the limited number of children reported in the literature with proven influenza infections would tend to under-represent the incidence of uncommon but serious complications such as death. Second, the preferential reporting of influenza outbreak seasons may over-estimate the importance of influenza. Future population based studies should focus on consecutive influenza seasons and capture indirect benefits of vaccinations such as interruption of transmission in the community so that preventive strategies for children can be reconsidered with special attention to their cost effectiveness.