Pandemic potential of a strain of influenza A (H1N1): early findings.
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.
MRC Centre for Outbreak Analysis and Modelling, Department of Infectious Disease Epidemiology, Imperial College London, Faculty of Medicine, Norfolk Place, London W2 1PG, UK.
Christophe
C
Fraser
Christl A
CA
Donnelly
Simon
S
Cauchemez
William P
WP
Hanage
Maria D
MD
Van Kerkhove
T Déirdre
TD
Hollingsworth
Jamie
J
Griffin
Rebecca F
RF
Baggaley
Helen E
HE
Jenkins
Emily J
EJ
Lyons
Thibaut
T
Jombart
Wes R
WR
Hinsley
Nicholas C
NC
Grassly
Francois
F
Balloux
Azra C
AC
Ghani
Neil M
NM
Ferguson
Andrew
A
Rambaut
Oliver G
OG
Pybus
Hugo
H
Lopez-Gatell
Celia M
CM
Alpuche-Aranda
Ietza Bojorquez
IB
Chapela
Ethel Palacios
EP
Zavala
Dulce Ma Espejo
DM
Guevara
Francesco
F
Checchi
Erika
E
Garcia
Stephane
S
Hugonnet
Cathy
C
Roth
D004196:Disease Outbreaks; D014943:Global Health; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000453:epidemiology* / Q000401:mortality / Q000635:transmission / Q000821:virology; D008800:Mexico / Q000453:epidemiology; D008969:Molecular Sequence Data; D014195:Travel