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American Journal of Epidemiology Advance Access published online on December 7, 2005
American Journal of Epidemiology, doi:10.1093/aje/kwj034
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American Journal of Epidemiology Copyright © 2005 by the Johns Hopkins Bloomberg School of Public Health All rights reserved; printed in U.S.A.
Received November 5, 2004
Accepted September 7, 2005

ORIGINAL CONTRIBUTIONS

Refined Estimate of the Incubation Period of Severe Acute Respiratory Syndrome and Related Influencing Factors

Quan-Cai Cai 1, Qin-Feng Xu 2, Jian-Ming Xu 3, Qiang Guo 4, Xiang Cheng 5, Gen-Ming Zhao 6, Qing-Wen Sun 5, Jian Lu 7, and Qing-Wu Jiang 6 *

1 Department of Epidemiology, School of Public Health, Fudan University, Shanghai, China; Department of Epidemiology, Second Military Medical University, Shanghai, China
2 Department of Statistics, Fudan University, Shanghai, China
3 Department of Organic Chemistry, Second Military Medical University, Shanghai, China
4 Department of Training, Second Military Medical University, Shanghai, China
5 Department of Mathematics and Physics, Second Military Medical University, Shanghai, China
6 Department of Epidemiology, School of Public Health, Fudan University, Shanghai, China
7 Department of Statistics, Second Military Medical University, Shanghai, China

* To whom correspondence should be addressed.
Qing-Wu Jiang, E-mail: qwjiang{at}shmu.edu.cn


  Abstract

Many epidemiologists have agreed that a refined estimate of the incubation period of severe acute respiratory syndrome (SARS) would need a sample size of about 200 cases and appropriate statistical methods enabling the inclusion of cases with defined periods of exposure. However, no such studies have been reported so far. Besides, determinants of the SARS incubation period remain unclear. In this study, 209 probable SARS cases with documented episodes of exposure between March 1 and May 31, 2003, in mainland China were included. A nonparametric method was used to analyze these data with defined periods of exposure to obtain the refined estimate of the SARS incubation period. Furthermore, the authors also explored the influence of various factors on the SARS incubation period by analysis of variance, linear regression analysis, and analysis of covariance. The estimates of mean and variance of the SARS incubation period were 5.29 days and 12.33 days2, respectively; 90% of patients would have an incubation period of less than 11.58 days with a probability of 0.8, and 99% of patients would have an incubation of less than 22.22 days with a probability of 0.9. The affected area showed a highly significant effect on the incubation period (p < 0.001), but the contact pattern, occupation, gender, and age did not.

Keywords: analysis of covariance; analysis of variance; regression analysis; severe acute respiratory syndrome; statistics, nonparametric.
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