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

19 June 2009   Vol 324, Issue 5934, Pages 1477-1602

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

Christophe Fraser,1,* Christl A. Donnelly,1,* 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. Alpuche-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

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 6000 to 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 1.8%) based on confirmed and suspected 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, although substantial uncertainty remains, clinical severity appears less than that seen in the 1918 influenza pandemic but comparable with that seen in the 1957 pandemic. Clinical attack rates in children in La Gloria were twice that in adults (<15 years of age: 61%; 15 years: 29%). Three different epidemiological analyses gave basic reproduction number (R0) estimates in the range of 1.4 to 1.6, whereas 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 that of seasonal flu, and comparable with lower estimates of R0 obtained from previous influenza pandemics.

1 MRC Centre for Outbreak Analysis and 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.

* These authors contributed equally to this work.
All authors are members of this collaboration.
To whom correspondence should be addressed. E-mail:

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