Email this article to a friend Respond to this article Other related articles in BMJ Download to Citation Manager Search Medline for articles by: Hsieh, Y.-H. || Chen, C. W.-S. Alert me when: New articles cite this article

Letter

Severe acute respiratory syndrome

Numbers do not tell whole story

The daily numbers of new cases of SARS in Taiwan up to 2 June have declined since mid-May (figure (top)). However, the delay caused by the incubation time as well as the time lag for diagnosis and reporting could result in subsequent under-reporting.

View larger version (28K): [in this window] [in a new window]   Top: Numbers of cases of SARS in Taiwan by date of onset (up to 2 June 2003; data from Centre for Disease Control of Taiwan's website (www.cdc.gov.tw/sarsen)). Bottom: Residuals of true values minus predicted values of cumulative number of cases of SARS fitted by exponential model

To ascertain the true direction of the epidemic, we fitted an exponential curve with series autocorrelation in the error structure to the cumulative numbers of cases of SARS in Taiwan obtained from the same dataset from 12 March to 25 May figure (bottom).

From 12 March to 19 April the model fits very well, showing exponential growth. From 20 April to 13 May, when a series of hospital outbreaks occurred,³ the data show wild variations and the predicted values consistently underestimate the true values.

Between 14 and 25 May the consistently downward trend of the real values shows that the increase in the cumulative number of cases of SARS is again exponential, albeit at a much slower rate. The stochastic variations from 20 April to 13 May, caused by a series of breakdowns in the health infrastructure, cannot be explained by a simple model. But the bottom figure offers definitive evidence that the growth of the epidemic is exponential but slowing down.

While the daily numbers in the top figure peak around 11-19 May—from the last major hospital cluster infections in the south of Taiwan during mid-May—the residuals in the bottom figure show a downward trend as early as 14 May.

This indicates that even with the last outbreak in a healthcare setting in the south, control measures had started to take effect. However, recent developments in Toronto warn about the possible consequence of a single misreporting, lest we let down our guard.

Ying-Hen Hsieh, professor

Department of Applied Mathematics, National Chung Hsing University, Taichung 402, Taiwan hsieh@amath.nchu.edu.tw

Cathy Woan-Shu Chen, professor

Graduate Institute of Statistics and Actuarial Science, Feng Chia University, Taichung 407

References

1. Parry J. Data show that SARS is gradually coming under control. BMJ 2003:326: 1166. (31 May.)[Free Full Text]
2. World Health Organization. Communicable Disease Surveillance and Control. Update 59—report on Guangxi (China) visit, situation in Taiwan, risk of SARS transmission during air travel. www.who.int/csr/sars/archive/2003_05_19/en/ (accessed 19 May 2003).
3. Hsieh YH. Politics hindering SARS work. Nature 2003;423: 381.

Other related articles in BMJ:

NEWS
Scottish royal colleges ask Blair to do more for Iraq's medical services.

Bryan Christie
BMJ 2003 326: 1166. [Full text]  

Email this article to a friend Respond to this article Other related articles in BMJ Download to Citation Manager Search Medline for articles by: Hsieh, Y.-H. || Chen, C. W.-S. Alert me when: New articles cite this article