Epidemiology of influenza in tropical and subtropical low-income regions

Influenza appears to be a major contributor to morbidity, hospitalization, and likely mortality in the tropical and subtropical low-income countries; however, its contribution has been largely underestimated due to a lack of data from these regions. Limited available data indicate that influenza circulation in the tropics differs in two respects from that in the temperate northern and southern hemispheres. First, while seasonal influenza tends to occur primarily in the late fall and winter in temperate zones, it appears to circulate year-round in tropics, with seasonal influenza A peaks between March and September in many of tropical settings, complementing temperate zone seasonality. This prolonged circulation may partly account for its apparent higher incidence in those countries where data are available. Virus circulation in East and Southeast Asia may determine seasonal reintroduction and circulation elsewhere. Second, the fraction of infections resulting in clinically important illness, particularly childhood pneumonia, appears to be higher in the tropics. Influenza may be responsible for a substantial fraction of the childhood pneumonia and pneumonia-related mortality, both from primary infection and from interaction with respiratory bacterial agents in the tropical belt. Introduction of influenza vaccine as a means to control influenza-related pneumonia in young children may be warranted. Indeed, control of childhood pneumonia may provide a mechanism for influenza vaccine uptake in these countries with wider benefits to both disease burden and mortality reduction, as well as surge capacity for vaccine production during pandemics. Concern about pandemic influenza has increased interest in vaccine use, including increased seasonal vaccine use, and initiation of vaccine production in some countries. Continued and enhanced surveillance in the tropics, particularly in East and Southeast Asia, is warranted both to monitor burden and the impact of interventions, such as vaccination, and to identify emergence and spread of novel viruses.