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Published Online May 11, 2009
Science DOI: 10.1126/science.1176062

Reports

Submitted on May 5, 2009
Accepted on May 11, 2009

Pandemic Potential of a Strain of Influenza A (H1N1) : Early Findings

Christophe Fraser 1{dagger}, Christl A. Donnelly 1{dagger}, Simon Cauchemez 1, William P. Hanage 1, Maria D. Van Kerkhove 1, T. Déirdre Hollingsworth 1, Jamie Griffin 1, Rebecca F. Baggaley 1, Helen E. Jenkins 1, Emily J. Lyons 1, Thibaut Jombart 1, Wes R. Hinsley 1, Nicholas C. Grassly 1, Francois Balloux 1, Azra C. Ghani 1, Neil M. Ferguson 1*, Andrew Rambaut 2, Oliver G. Pybus 3, Hugo Lopez-Gatell 4, Celia M Apluche-Aranda 5, Ietza Bojorquez Chapela 4, Ethel Palacios Zavala 4, Dulce Ma. Espejo Guevara 6, Francesco Checchi 7, Erika Garcia 7, Stephane Hugonnet 7, Cathy Roth 7, The WHO Rapid Pandemic Assessment Collaboration {ddagger}

1 MRC Centre for Outbreak Analysis & Modelling, Department of Infectious Disease Epidemiology, Imperial College London, Faculty of Medicine, Norfolk Place, London W2 1PG, UK.
2 Institute of Evolutionary Biology, University of Edinburgh, Ashworth Laboratories Edinburgh EH9 3JT, UK.
3 Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK.
4 Directorate General of Epidemiology, FCO. De P. Miranda 177 5th Floor, Mexico City, 01480, Mexico.
5 National Institute of Epidemiological Diagnosis and Reference, Prolongación Carpio No. 470 (3° piso), Col Santo Tomás, México City, C.P. 11340, Mexico.
6 Secretaría de Salud - Servicios de Salud de Veracruz Soconusco No. 36 Colonia Aguacatal C.P. 910 Xalapa, Veracruz, México State.
7 World Health Organization, 20 Av. Appia, 1211 Geneva, Switzerland.

* To whom correspondence should be addressed.
Neil M. Ferguson , E-mail: neil.ferguson{at}imperial.ac.uk

{dagger}These authors contributed equally to this work.

{ddagger}All authors are members of this collaboration.

A novel influenza A (H1N1) virus has spread rapidly across the globe. Judging its pandemic potential is difficult with limited data, but nevertheless essential to inform appropriate health responses. By analyzing the outbreak in Mexico, early data on international spread, and viral genetic diversity, we make an early assessment of transmissibility and severity. Our estimates suggest that 23,000 (range 6,000-32,000) individuals had been infected in Mexico by late April, giving an estimated case fatality ratio (CFR) of 0.4% (range 0.3% to 1.5%) based on confirmed and suspect deaths reported to that time. In a community outbreak in the small community of La Gloria, Veracruz no deaths were attributed to infection, giving an upper 95% bound on CFR of 0.6%. Thus while substantial uncertainty remains, clinical severity appears less than that seen in 1918 but comparable with that seen in 1957. Clinical attack rates in children in La Gloria were twice that in adults (<15 years-of-age: 61%, ≥15: 29%). Three different epidemiological analyses gave R0 estimates in the range 1.4-1.6, while a genetic analysis gave a central estimate of 1.2. This range of values is, consistent with 14 to 73 generations of human-to-human transmission having occurred in Mexico to late April. Transmissibility is therefore substantially higher than seasonal flu, and comparable with lower estimates of R0 obtained from previous influenza pandemics.


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