Many research questions about influenza in tropical regions remain unanswered. For instance, Hong Kong is a wealthy subtropical city, with a higher per-capita gross domestic product than Australia or New Zealand. Thus, the present Hong Kong study cannot inform us about the impact of influenza in poor tropical countries, where the severity of infections is likely compounded by malnutrition, limited supplies of antibiotics to treat secondary bacterial infections, and limited access to hospital care.
Modeling the influenza burden in tropical countries relies on good-quality virus surveillance data. And because of large year-to-year variations in the impact of influenza, modeling also depends on the availability of several years' worth of data. Since influenza surveillance efforts were only recently initiated in tropical countries, studies of the influenza burden are generally of short duration (five years in the study by Peiris et al. [
]). The information generated so far during a period mostly dominated by more pathogenic influenza A (H3N2) viruses may not reflect the average influenza experience in Hong Kong [
]. An additional shortcoming of the virus-driven modeling approach is that it cannot study past epidemics and pandemics for which no viral surveillance data was collected. Furthermore in this approach, deaths or hospitalizations attributed to influenza are directly proportional to the intensity of virus activity, and therefore, week-to-week variations in virus isolation rates should truly reflect changes in disease prevalence and not sampling error. Laboratory surveillance needs to be strengthened before modeling of disease burden becomes feasible in most tropical countries [
Finally, more studies are needed to elucidate influenza seasonal patterns across a large range of latitudes, representing several tropical countries in both hemispheres. Of particular interest is the long-standing question of persistence: does influenza persist in temperate countries all year long (through undetectable chains of transmission in summer) or is it reintroduced from the Tropics each year at the beginning of winter? A combination of enhanced virus surveillance and sequence analysis of the viral genome of circulating strains is an important avenue for future research to address this question, and Hong Kong may be an essential sampling site in this respect.
Resolving this question is of paramount importance to vaccine composition and influenza control in tropical regions. Because of year-round influenza activity, it is not clear whether the Northern Hemisphere or Southern Hemisphere recommended composition should be used, and what the optimal timing for the vaccination campaign would be. Indeed, the influenza vaccination strategy may have to be tailored to each individual tropical region, based on local prevalence, timing, cycling of influenza strains, and how they match the strains circulating in temperate areas.