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Originally published in Science Express on 23 May 2003
Science 20 June 2003:
Vol. 300. no. 5627, pp. 1961 - 1966
DOI: 10.1126/science.1086478
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Reports

Transmission Dynamics of the Etiological Agent of SARS in Hong Kong: Impact of Public Health Interventions

Steven Riley,1*{dagger} Christophe Fraser,1* Christl A. Donnelly,1 Azra C. Ghani,1 Laith J. Abu-Raddad,1 Anthony J. Hedley,2 Gabriel M. Leung,2 Lai-Ming Ho,2 Tai-Hing Lam,2 Thuan Q. Thach,2 Patsy Chau,2 King-Pan Chan,2 Su-Vui Lo,3 Pak-Yin Leung,4 Thomas Tsang,4 William Ho,5 Koon-Hung Lee,5 Edith M. C. Lau,6 Neil M. Ferguson,1 Roy M. Anderson1

We present an analysis of the first 10 weeks of the severe acute respiratory syndrome (SARS) epidemic in Hong Kong. The epidemic to date has been characterized by two large clusters—initiated by two separate "super-spread" events (SSEs)—and by ongoing community transmission. By fitting a stochastic model to data on 1512 cases, including these clusters, we show that the etiological agent of SARS is moderately transmissible. Excluding SSEs, we estimate that 2.7 secondary infections were generated per case on average at the start of the epidemic, with a substantial contribution from hospital transmission. Transmission rates fell during the epidemic, primarily as a result of reductions in population contact rates and improved hospital infection control, but also because of more rapid hospital attendance by symptomatic individuals. As a result, the epidemic is now in decline, although continued vigilance is necessary for this to be maintained. Restrictions on longer range population movement are shown to be a potentially useful additional control measure in some contexts. We estimate that most currently infected persons are now hospitalized, which highlights the importance of control of nosocomial transmission.

1 Department of Infectious Disease Epidemiology, Faculty of Medicine, Imperial College London, Exhibition Road, London SW7 2AZ, UK.
2 Department of Community Medicine, University of Hong Kong, 21 Sassoon Road, Pokfulam, Hong Kong.
3 Research Office, Health, Welfare and Food Bureau, Government of the Hong Kong Special Administrative Region, 19th Floor, Murray Building, Garden Road, Hong Kong.
4 Department of Health, Government of the Hong Kong Special Administrative Region, Wu Chung House, 213 Queen's Road East, Wanchai, Hong Kong.
5 Hong Kong Hospital Authority, 147B Argyle Street, Kowloon, Hong Kong.
6 Department of Community and Family Medicine, Chinese University of Hong Kong, School of Public Health, Prince of Wales Hospital, Shatin, N.T., Hong Kong.



* These authors contributed equally to this work.

{dagger} To whom correspondence should be addressed. E-mail: s.riley{at}imperial.ac.uk

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