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 R 0 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.
Highlights
In late April 2009, the first cases of the novel swine-derived H1N1pdm influenza A virus were detected in Mexico and the United States, prompting the World Health Organization (WHO) to raise the level of influenza pandemic alert to phase 5 [1]
This period is outside the typical influenza season in temperate countries in the Northern hemisphere, but in the typical winter season for influenza transmission for countries from temperate regions of the Southern Hemisphere
The 2009 H1N1 pandemic reminded us once again of the on-going threat posed by novel respiratory viruses and the need for understanding better how such pathogens emerge and spread
Summary
In late April 2009, the first cases of the novel swine-derived H1N1pdm influenza A virus were detected in Mexico and the United States, prompting the World Health Organization (WHO) to raise the level of influenza pandemic alert to phase 5 [1]. By the end of 2009, the H1N1pdm virus had spread to more than 208 countries, resulting in hundreds of thousands of cases and at least 18000 deaths [2,3]. As a result the H1N1pdm is one of the most documented pandemics with enhanced surveillance established in many regions of the globe, with the exception of Africa [5,6]. The H1N1pdm virus was introduced into most northern and southern hemisphere countries during the spring and summer of. In most Southern hemisphere temperate countries, a full epidemic of H1N1pdm influenza was observed and the pandemic strain quickly became the predominant circulating influenza virus, replacing seasonal strains in many countries [7]
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