BMJ  2003;326:1396 (21 June), doi:10.1136/bmj.326.7403.1396

Letter

Severe acute respiratory syndrome

Capture-recapture method should be used to count how many cases of SARS really exist

EDITOR—Parry suggested that the epidemic of the severe acute respiratory syndrome (SARS) is beginning to slow.¹ This is probably true as cold and influenza viruses naturally dissipate during the summer (nonflu season²). This observed reduction would include the coronaviruses, which have been estimated to result in around 30% of the reported cases of the common cold. So as the summer progresses, there should be a reduced number of cases occurring, although the disease may not completely vanish, especially for those in an occupational setting and rural areas.³

The real question being asked is: How many cases have actually occurred? This is important since these rates will indicate whether control measures are effective and the disease is dissipating. Many of the rural areas of China and most likely other locations are not being monitored for rates of SARS, and these locations may act as reservoirs. The number of cases reported cannot include the undercounts, or missed cases.

A method that has not been applied for estimating the number of SARS cases is capture-recapture.⁴ The capture-recapture method uses two or more lists for estimating the number of cases.⁴ These lists can be easily obtained from hospitals, health departments, surveys, and private practitioners and they do not require complete ascertainment.⁵ This method can be applied to a region, nation, or the entire world and will also provide an estimate of the undercounts.⁵

The capture-recapture method should be considered the gold standard for counting when it is impossible to identify each case and large undercounts will occur.

Envirosafe Training and Consultants, PO Box 114022, Pittsburgh, PA 15239, USA john.pam.lange{at}worldnet.att.net

Department of Epidemiology, Graduate School of Public Health, University of Pittsburgh, 3512 Fifth Avenue, Pittsburgh, PA 15213, USA

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Competing interests: None declared.

References

1. Parry J. WHO is worried that China is under-reporting SARS. BMJ 2003:326: 1110. (24 May.)[Free Full Text]
2. Neuzil KM, Hohlbein C, Zhu Y. Illness among schoolchildren during influenza season: effect on school absenteeism, parental absenteeism from work, and secondary illness in families. Arch Pediatr Adolesc Med 2002;156: 986-91.[Abstract/Free Full Text]
3. Lange JH. The best protection. Can Med Assoc J 2003;168: 1524.[Free Full Text]
4. LaPorte RE, McCarty DJ, Tull ES, Tajima N. Counting birds, bees and NCDs. Lancet 1992;339: 494-5.[ISI][Medline]
5. International Working Group for Disease Monitoring and Forecasting. Capture-recapture and multiple-record systems estimation. I. History and theoretical development. Am J Epidemiol 1995:142: 1047-58.[Abstract/Free Full Text]

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