Differential age-specific distribution of influenza virus types and subtypes in tropical Singapore, 2011-2017

Surveillance and reporting of epidemiological features of seasonal influenza mostly are aggregates across all-ages. We investigated age-specific differences in distribution of influenza virus (sub)types in tropical Singapore, using laboratory-confirmed virological data collected under the national influenza surveillance programme in 2011-2017. The proportion of influenza-positive specimens from outpatients with influenza-like illness was used as an indicator of influenza activity in the community. The highest influenza positivity for age groups of 5-14 years and 15-64 years coincided in the same month in five out of the seven years under study. Influenza positivity was lowest in young children <5 years of age compared to older age groups. Influenza A(H3N2) was most prevalent in the community except in 2012 when a predominance of influenza B was observed. The dominant virus (sub)type varied across the years in children <5 years and 5-14 years of age. Influenza A(H3N2) was the predominant circulating virus subtype among elderly persons aged ≥65 years during the seven-year period, and among adults aged 15-64 years since 2013. Knowledge about the age-specific differences in distribution of influenza virus (sub)types helps to facilitate better understanding of seasonal epidemics and to inform targeted strategies in prevention and control of influenza virus transmission.