Global Patterns in Seasonal Activity of Influenza A/H3N2, A/H1N1, and B from 1997 to 2005: Viral Coexistence and Latitudinal Gradients

Brian S Finkelman,Matthew J Ferrari,Katia Koelle,Cécile Viboud,Nita Bharti,Bryan T Grenfell,Landon Myer

doi:10.1371/journal.pone.0001296

Abstract

Despite a mass of research on the epidemiology of seasonal influenza, overall patterns of infection have not been fully described on broad geographic scales and for specific types and subtypes of the influenza virus. Here we provide a descriptive analysis of laboratory-confirmed influenza surveillance data by type and subtype (A/H3N2, A/H1N1, and B) for 19 temperate countries in the Northern and Southern hemispheres from 1997 to 2005, compiled from a public database maintained by WHO (FluNet). Key findings include patterns of large scale co-occurrence of influenza type A and B, interhemispheric synchrony for subtype A/H3N2, and latitudinal gradients in epidemic timing for type A. These findings highlight the need for more countries to conduct year-round viral surveillance and report reliable incidence data at the type and subtype level, especially in the Tropics.

Highlights

Influenza is a major human pathogen, the epidemiology of which is characterized by epidemics that occur seasonally throughout the world every year, with occasional pandemics arising from novel subtypes of the virus; both annual and pandemic influenza are the source of considerable morbidity, mortality, and economic burden [1]
Because influenza epidemics typically occur during winter months, a ‘‘season’’ in the Northern Hemisphere was defined as occurring from week 27 of one calendar year to week 26 of the following calendar year, so that one defined ‘‘season’’ would include an entire influenza epidemic; a Southern Hemisphere ‘‘season’’ was defined as being the same as the calendar year
Through a quantitative analysis of laboratory-confirmed weekly type- and subtype-specific FluNet data from 19 countries, we found both support for previously-held seasonal and type and subtype dominance patterns [1,2,17,19] as well as novel patterns of interhemispheric synchrony and latitudinal gradients in epidemic timing

Summary

Introduction

Influenza is a major human pathogen, the epidemiology of which is characterized by epidemics that occur seasonally throughout the world every year, with occasional pandemics arising from novel subtypes of the virus; both annual and pandemic influenza are the source of considerable morbidity, mortality, and economic burden [1]. Antigenic evolution in influenza A is punctuated rather than continuous, characterized by the emergence of clusters of antigenically similar but genetically unique strains that dominate subtype incidence [8]. They carry distinct surface antigens, H3 and H1 have been shown to provide some level of cross-immunity to each other [9,10]. Interference competition for susceptible hosts may occur between the two subtypes, such that one subtype could potentially influence the dynamics of the other [11]

Methods

Results

Conclusion