The Transmissibility and Control of Pandemic Influenza A (H1N1) Virus

Abstract

Pandemic influenza A (H1N1) 2009 (pandemic H1N1) is spreading throughout the planet. It has become the dominant strain in the southern hemisphere, where the influenza season is underway. Here, based on reported case clusters in the USA, we estimate the household secondary attack rate for pandemic H1N1 to be 27.3% (95% CI: 12.2%-50.5%). From a school outbreak, we estimate a school child infects 2.4 (95% CI: 1.8-3.2) other children within the school. We estimate the basic reproductive number, R 0 , to range from 1.3-1.7 and the generation interval to range from 2.6-3.2 days. We use a simulation model to evaluate the effectiveness of vaccination strategies in the USA for the Fall, 2009. If vaccine were available soon enough, vaccination of children, followed by adults, reaching 70% overall coverage, in addition to high risk and essential workforce groups, could mitigate a severe epidemic.Pandemic H1N1, which first emerged in Mexico in , spread worldwide, resulting in more than 130,000 laboratory-confirmed cases and 800 deaths in over 100 countries by midJuly (1). The global distribution of this novel strain prompted the World Health Organization to declare the first influenza pandemic of the 21st century in June 2009 (2). Initially, most cases were clustered in households (3-6) and schools (7) with over 50% of the reported cases in school children in the 5-18 year old age range. A recent analysis of data from the United States, Canada, the United Kingdom, and the European Union suggests case fatality ratios ranging from 0.20%-0.68% in these regions and a higher case fatality ratio in Mexico of 1.23% (95% CI 1.03%-1.47%) (8).Both pandemic and seasonal influenza cause sustained epidemics in the upper northern hemisphere (above latitude ~ 20°N) and lower southern hemisphere (below latitude ~ 20°S) during the respective late Fall to early Spring months, with epidemics in the more tropical regions (between latitudes ~ 20°S and 20°N) occurring sporadically, but sometimes corresponding to the rainy season. The last influenza pandemic was the Hong Kong A (H3N2) * To whom correspondence should be addressed. longini@scharp.org . 1957 -1958 , caused mid-Summer, 1957, outbreaks in Louisiana schools that were open in the Summer because of the need for children helping with the Spring harvest (11). However, there was no extensive community-wide spread of influenza A (H2N2) in the USA until the Fall of 1957, with the national level epidemic rising in September and peaking in October. Pandemic H1N1 will probably spread in a similar spatio-temporal pattern as previous pandemics, but accelerated due to increased air travel (12). Supporting Online MaterialEstimates of the transmissibility of pandemic H1N1are crucial to devising effective mitigation strategies. Historically, the best characterization of influenza transmissibility has been based on the household secondary attack rate. The household secondary attack rate is the probability (sometimes expressed as a percent) that an infected person in the household will infect a...

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Influenza A: From highly pathogenic H5N1 to pandemic 2009 H1N1. Epidemiology and clinical features

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Crystal structure of the swine-origin A (H1N1)-2009 influenza A virus hemagglutinin (HA) reveals similar antigenicity to that of the 1918 pandemic virus

“…Current S-OIV (2009 H1N1 influenza virus) is a triple reassortment (in terms of host-origin) of influenza gene segments, with PA and PB2 from avian, PB1 from human and HA, NP, NS, NA and M from classical/Eurasian swine (Dawood et al, 2009;Smith et al, 2009;Gao and Sun, 2010). This current H1N1 virus has been found to have many similar properties to the 1918 pandemic H1N1 virus, including the overall similarity of the genome and characteristics of pathogenesis (Itoh et al, 2009;Shen et al, 2009;WHO, 2009;Yang et al, 2009;Igarashi et al, 2010;Yeping Sun et al, 2010).…”