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Pandemic Potential of a Strain of Influenza A (H1N1): Early Findings
Christophe Fraser,1,*Christl A. Donnelly,1,*Simon Cauchemez,1William P. Hanage,1Maria D. Van Kerkhove,1T. Déirdre Hollingsworth,1Jamie Griffin,1Rebecca F. Baggaley,1Helen E. Jenkins,1Emily J. Lyons,1Thibaut Jombart,1Wes R. Hinsley,1Nicholas C. Grassly,1Francois Balloux,1Azra C. Ghani,1Neil M. Ferguson,1,Andrew Rambaut,2Oliver G. Pybus,3Hugo Lopez-Gatell,4Celia M. Alpuche-Aranda,5Ietza Bojorquez Chapela,4Ethel Palacios Zavala,4Dulce Ma. Espejo Guevara,6Francesco Checchi,7Erika Garcia,7Stephane Hugonnet,7Cathy Roth,7The WHO Rapid Pandemic Assessment Collaboration
A novel influenza A (H1N1) virus has spread rapidly across theglobe. Judging its pandemic potential is difficult with limiteddata, but nevertheless essential to inform appropriate healthresponses. By analyzing the outbreak in Mexico, early data oninternational spread, and viral genetic diversity, we make anearly assessment of transmissibility and severity. Our estimatessuggest that 23,000 (range 6000 to 32,000) individuals had beeninfected in Mexico by late April, giving an estimated case fatalityratio (CFR) of 0.4% (range: 0.3 to 1.8%) based on confirmedand suspected deaths reported to that time. In a community outbreakin the small community of La Gloria, Veracruz, no deaths wereattributed to infection, giving an upper 95% bound on CFR of0.6%. Thus, although substantial uncertainty remains, clinicalseverity appears less than that seen in the 1918 influenza pandemicbut comparable with that seen in the 1957 pandemic. Clinicalattack rates in children in La Gloria were twice that in adults(<15 years of age: 61%; 15 years: 29%). Three different epidemiologicalanalyses gave basic reproduction number (R0) estimates in therange of 1.4 to 1.6, whereas a genetic analysis gave a centralestimate of 1.2. This range of values is consistent with 14to 73 generations of human-to-human transmission having occurredin Mexico to late April. Transmissibility is therefore substantiallyhigher than that of seasonal flu, and comparable with lowerestimates 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: neil.ferguson{at}imperial.ac.uk
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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. 
All authors are members of this collaboration. 
To whom correspondence should be addressed. E-mail: 
