Incidence of laboratory-confirmed influenza disease among infants under 6 months of age: a systematic review

OBJECTIVES: The aim of this systematic review was to assess incidence rates of laboratory-confirmed influenza (LCI) outcomes among infants under 6 months of age. DESIGN: Systematic literature search and review of indexed studies in PubMed, EMBASE, the Cochrane Library and CINAHL Plus from inception to 19 April 2017. SETTING: Population-based estimates from community or hospital settings. PARTICIPANTS: Infants under 6 months of age. PRIMARY AND SECONDARY OUTCOME MEASURES: LCI illness in ambulatory care settings, LCI hospitalisation, LCI intensive care unit admission and LCI death. Only studies with population-based incidence data were included. RESULTS: We identified 27 primary studies, 11 of which were from the USA, four were from other non-US high-income settings and the remaining were from lower-middle-income or upper-middle-income countries. Most studies (n=23) assessed incidence of LCI hospitalisation, but meta-analysis to pool study-specific rates was not possible due to high statistical and methodological heterogeneity. Among US studies, the reported incidence of LCI hospitalisation ranged from 9.3 to 91.2 per 10 000 infants under 6 months for seasonal influenza, while the only US-based estimate for pandemic H1N1 influenza was 20.2 per 10 000 infants. Reported rates for LCI hospitalisation for seasonal influenza from other countries ranged from 6.2 to 73.0 per 10 000 infants under 6 months, with the exception of one study with an estimated rate of 250 per 10 000 infants. No events were reported in five of the nine studies that evaluated LCI death among infants under 6 months. CONCLUSION: Our review of published studies found limited data on LCI outcomes for infants under 6 months, particularly from non-US settings. Globally representative and reliable incidence data are necessary to fully evaluate influenza disease burden and the potential impact of maternal influenza immunisation programme on morbidity and mortality in young infants.