We estimated the incidence of influenza episodes, influenza-associated acute lower respiratory infections (ALRI), and influenza-associated severe ALRI in children younger than 5 years, stratified by age, with data from a systematic review of studies published between Jan 1, 1995, and Oct 31, 2010, and 16 unpublished population-based studies. We applied these incidence estimates to global population estimates for 2008 to calculate estimates for that year. We estimated possible bounds for influenza-associated ALRI mortality by combining incidence estimates with case fatality ratios from hospital-based reports and identifying studies with population-based data for influenza seasonality and monthly ALRI mortality.
We identified 43 suitable studies, with data for around 8 million children. We estimated that, in 2008, 90 million (95 % CI 49?62 million) new cases of influenza (data from nine studies), 20 million (13?2 million) cases of influenza-associated ALRI (13 % of all cases of paediatric ALRI; data from six studies), and 1 million (1? million) cases of influenza-associated severe ALRI (7 % of cases of all severe paediatric ALRI; data from 39 studies) occurred worldwide in children younger than 5 years. We estimated there were 28?00?11?00 deaths in children younger than 5 years attributable to influenza-associated ALRI in 2008, with 99 % of these deaths occurring in developing countries. Incidence and mortality varied substantially from year to year in any one setting.
Influenza is a common pathogen identified in children with ALRI and results in a substantial burden on health services worldwide. Sufficient data to precisely estimate the role of influenza in childhood mortality from ALRI are not available.
WHO; Bill & Melinda Gates Foundation.