%0 Journal Article
%T Transmission Characteristics of the 2009 H1N1 Influenza Pandemic: Comparison of 8 Southern Hemisphere Countries
%A Lulla Opatowski
%A Christophe Fraser
%A Jamie Griffin
%A Eric de Silva
%A Maria D. Van Kerkhove
%A Emily J. Lyons
%A Simon Cauchemez
%A Neil M. Ferguson
%J PLOS Pathogens
%D 2011
%I Public Library of Science (PLoS)
%R 10.1371/journal.ppat.1002225
%X 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, R0, 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¨C1.8 for the country-specific fits, and 1.29¨C1.47 for the global fits. Corresponding estimates for overall attack-rate were in the range 20¨C50%. 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 ¨C 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.
%U http://www.plospathogens.org/article/info%3Adoi%2F10.1371%2Fjournal.ppat.1002225