Citation: Viboud C, Alonso WJ, Simonsen L (2006) Influenza in Tropical Regions. PLoS Med 3(4): e89. https://doi.org/10.1371/j
Citation: Viboud C, Alonso WJ, Simonsen L (2006) Influenza in Tropical Regions. PLoS Med 3(4): e89. https://doi.org/10.1371/journal.pmed.0030089
Published: March 7, 2006
This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.
Funding: The Fogarty International Center (National Institutes of Health) provided funding for this article.
Competing interests: The authors declare that no competing interests exist.
The global burden of influenza on morbidity and mortality is considerable, with an estimated 1 million annual deaths worldwide. In temperate regions, there are clear seasonal variations in the occurrence of influenza, with a marked peak in cold winter months. In contrast, seasonality is less defined in tropical regions—here, there is high background influenza activity throughout the year, on top of which epidemics occur at intermediate months between the influenza season in temperate countries of the Northern and Southern hemispheres. Influenza seasonal patterns in the Americas are summarized in Figure 1, which illustrates the transition between the Northern and Southern hemispheres. Very little is known about exactly how and why the seasonality of influenza varies with latitude. No link with environmental conditions has been convincingly shown, although influenza activity appears to coincide with the rainy season in some tropical countries.
(A) Weekly distribution of influenza laboratory isolates (from week 1 to week 52; weekly frequency ( y-axis) is calculated as the weekly number of isolates divided by the annual number of isolates).
(B) Weekly numbers of isolates were aggregated over four-week periods to show a (nearly) monthly distribution of influenza circulation. Each color represents a different month (color bar on the right). Note the transition in seasonal patterns from north to south, ranging from marked seasonal winter activity centered around January in the US, to uniform circulation throughout the year in Columbia and again, strong winter epidemics center around July in Argentina.
Influenza viral surveillance data compiled from WHO Flunet.
Until very recently, the impact of influenza on health was believed to be insignificant in tropical regions. But this impression may be changing, in part thanks to the recent studies by Joseph S. M. Peiris and his colleagues from Hong Kong, one of which appears in this issue of PLoS Medicine. Hong Kong is a subtropical city located within the likely epicenter of pandemic influenza in Southeast Asia.
Methods for Estimating the Burden of Influenza
Estimating the burden of influenza on hospitalizations and deaths is not straightforward in temperate countries, and it is even less so in tropical countries. Severe clinical outcomes are often caused by secondary bacterial infections, and a primary influenza infection may be unrecognized. Additionally, laboratory confirmation of influenza infection is rarely conducted. As a result, most influenza-related hospitalizations and deaths are not attributed to influenza on discharge forms and death certificates, and statistical methods are necessary to assess the true burden of influenza on health.
Traditionally, for temperate countries, the impact of a particular influenza epidemic is estimated as the number of “excess deaths” or “excess cases” above a seasonal baseline. This approach relies on identifying periods without influenza activity to generate a model baseline of expected winter deaths (or cases) [6–8]. But the approach fails in tropical regions, where there is no well-defined non-influenza period (Figure 1). An alternative method recently developed by the United States Centers for Disease Control uses virus surveillance data to guide models of burden, assuming a proportional relation between virus activity and the number of hospitalization or deaths attributable to influenza [9, 10]. For the US, both traditional and new methods produced very similar estimates of influenza mortality burden (Table 1 and [11, 12]). In their Hong Kong study, Peiris et al. appropriately used a variation of the virus-guided modeling approach by linking weekly hospital discharges with weekly prevalence of influenza virus in respiratory specimens sampled in the community.
Influenza Burden is Broadly Similar in Hong Kong and the US
Peiris et al. found that the annual influenza-related hospitalization rates are quite similar in Hong Kong and temperate countries (Table 1), confirming their