title: Transmission characteristics of the 2009 H1N1 influenza pandemic: comparison of 8 Southern hemisphere countries.
abstract: While in Northern hemisphere countries, the pandemic H1N1 virus (H1N1pdm) was introduced outside of the typical influenza season, Southern hemisphere countries experienced a single wave of transmission during their 2009 winter season. This provides a unique opportunity to compare the spread of a single virus in different countries and study the factors influencing its transmission. Here, we estimate and compare transmission characteristics of H1N1pdm for eight Southern hemisphere countries/states: Argentina, Australia, Bolivia, Brazil, Chile, New Zealand, South Africa and Victoria (Australia). Weekly incidence of cases and age-distribution of cumulative cases were extracted from public reports of countries' surveillance systems. Estimates of the reproduction numbers, R(0), empirically derived from the country-epidemics' early exponential phase, were positively associated with the proportion of children in the populations (p = 0.004). To explore the role of demography in explaining differences in transmission intensity, we then fitted a dynamic age-structured model of influenza transmission to available incidence data for each country independently, and for all the countries simultaneously. Posterior median estimates of R₀ ranged 1.2-1.8 for the country-specific fits, and 1.29-1.47 for the global fits. Corresponding estimates for overall attack-rate were in the range 20-50%. All model fits indicated a significant decrease in susceptibility to infection with age. These results confirm the transmissibility of the 2009 H1N1 pandemic virus was relatively low compared with past pandemics. The pattern of age-dependent susceptibility found confirms that older populations had substantial--though partial--pre-existing immunity, presumably due to exposure to heterologous influenza strains. Our analysis indicates that between-country-differences in transmission were at least partly due to differences in population demography.
authors: MRC Centre for Outbreak Analysis and Modelling, Department of Infectious Disease Epidemiology, School of Public Health, Imperial College London, London, United Kingdom. l.opatowski@imperial.ac.uk
authors: Lulla
authors: L
authors: Opatowski
authors: Christophe
authors: C
authors: Fraser
authors: Jamie
authors: J
authors: Griffin
authors: Eric
authors: E
authors: de Silva
authors: Maria D
authors: MD
authors: Van Kerkhove
authors: Emily J
authors: EJ
authors: Lyons
authors: Simon
authors: S
authors: Cauchemez
authors: Neil M
authors: NM
authors: Ferguson
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D001118:Argentina / Q000453:epidemiology; D001315:Australia / Q000453:epidemiology; D001838:Bolivia / Q000453:epidemiology; D001938:Brazil / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D002677:Chile / Q000453:epidemiology; D006801:Humans; D015994:Incidence; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000453:epidemiology / Q000635:transmission*; D008875:Middle Aged; D015233:Models, Statistical; D009520:New Zealand / Q000453:epidemiology; D058873:Pandemics / Q000706:statistics & numerical data; D013019:South Africa / Q000453:epidemiology; D014739:Victoria / Q000453:epidemiology