Transmission characteristics of the 2009 H1N1 influenza pandemic: comparison of 8 Southern hemisphere countries.

TitleTransmission characteristics of the 2009 H1N1 influenza pandemic: comparison of 8 Southern hemisphere countries.
Publication TypeJournal Article
Year of Publication2011
AuthorsOpatowski L, Fraser C, Griffin J, de Silva E, Van Kerkhove MD, Lyons EJ, Cauchemez S, Ferguson NM
JournalPLoS Pathog
Volume7
Issue9
Paginatione1002225
Date Published2011 Sep
ISSN1553-7374
KeywordsAdolescent, Adult, Age Factors, Aged, Argentina, Australia, Bolivia, Brazil, Child, Child, Preschool, Chile, Humans, Incidence, Infant, Influenza A Virus, H1N1 Subtype, Influenza, Human, Middle Aged, Models, Statistical, New Zealand, Pandemics, South Africa, Victoria
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.

DOI10.1371/journal.ppat.1002225
Alternate JournalPLoS Pathog.
PubMed ID21909272
PubMed Central IDPMC3164643
Grant ListG0800596 / / Medical Research Council / United Kingdom
U54 GM088491 / GM / NIGMS NIH HHS / United States
/ / Medical Research Council / United Kingdom
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