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The Effective Reproduction Number of Pandemic Influenza: Prospective Estimation

Cowling, Benjamin J.a; Lau, Max S. Y.a; Ho, Lai-Minga; Chuang, Shuk-Kwanb; Tsang, Thomasb; Liu, Shao-Haeic; Leung, Pak-Yinc; Lo, Su-Vuic,d; Lau, Eric H. Y.a

doi: 10.1097/EDE.0b013e3181f20977
Infectious Disease: Brief Report

Background: Timely estimation of the transmissibility of a novel pandemic influenza virus was a public health priority in 2009.

Methods: We extended methods for prospective estimation of the effective reproduction number (Rt) over time in an emerging epidemic to allow for reporting delays and repeated importations. We estimated Rt based on case notifications and hospitalizations associated with laboratory-confirmed pandemic (H1N1) 2009 virus infections in Hong Kong from June through October 2009.

Results: Rt declined from around 1.4–1.5 at the start of the local epidemic to around 1.1–1.2 later in the summer, suggesting changes in transmissibility perhaps related to school vacations or seasonality. Estimates of Rt based on hospitalizations of confirmed H1N1 cases closely matched estimates based on case notifications.

Conclusion: Real-time monitoring of the effective reproduction number is feasible and can provide useful information to public health authorities for situational awareness and calibration of mitigation strategies.

From the aInfectious Disease Epidemiology Group, School of Public Health, The University of Hong Kong, Hong Kong Special Administrative Region, China; bDepartment of Health, Centre for Health Protection, Government of the Hong Kong Special Administrative Region, People's Republic of China; cHospital Authority, Hong Kong Special Administrative Region, People's Republic of China; and dFood and Health Bureau, Government of the Hong Kong Special Administrative Region, People's Republic of China.

Submitted 11 December 2009; accepted 21 June 2010; posted 30 August 2010.

Supported by the Research Fund for the Control of Infectious Disease, Food and Health Bureau, Government of the Hong Kong SAR (HK-09–04–02), the Harvard Center for Communicable Disease Dynamics from the US National Institutes of Health Models of Infectious Disease Agent Study program (grant 1 U54 GM088558), and the Area of Excellence Scheme of the Hong Kong University Grants Committee (AoE/M-12/06).

Supplemental digital content is available through direct URL citations in the HTML and PDF versions of this article (www.epidem.com).

Correspondence: Benjamin J. Cowling, School of Public Health, The University of Hong Kong, Units 624–7, Core F, Cyberport 3, Pokfulam, Hong Kong. E-mail: bcowling@hku.hk.

© 2010 Lippincott Williams & Wilkins, Inc.