a Department of Epidemiology and Preventive Health Sciences, School of Health Sciences and Nursing, University of Tokyo, Hongo 731, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail hasimoto{at}epistat.m.u-tokyo.ac.jp
b Division of Health Informatics and Biostatistics, Oita University of Nursing and Health Sciences, Japan.
c Division of Intelligence and Policies, Infectious Disease Surveillance Centre, National Institute of Infectious Diseases, Japan.
d Department of Public Health, Saitama Medical School, Japan.
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Abstract |
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Methods We used as an index the number of cases per week per sentinel medical institution in the area covered by a health centre in infectious disease surveillance in Japan in 19931997. Periods of epidemics in health centre areas were determined according to the reported indices. The simple method used for detecting the early stage of an epidemic is that if the index exceeds a critical value, then an epidemic will begin in the following 4 weeks. The sensitivity, specificity and positive predictive value for this epidemic warning were evaluated for given critical values.
Results When the specificity of the epidemic warning was more than 95%, the sensitivity was more than 60% in ten diseases, and more than 80% in four diseases (influenza-like illness, rubella, hand-foot-and-mouth disease, and herpangina). The positive predictive value was between 15.6% and 31.4% in these ten diseases.
Conclusion The early stage of epidemics of some infectious diseases might be detectable using this simple method.
Keywords Infectious disease, surveillance, epidemic
Accepted 13 April 2000
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Introduction |
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Many epidemiological and statistical studies have investigated the detection of epidemics through surveillance.2,5,1115 These investigations have mainly focused on epidemics of influenzalike illnesses in a large area (whole country), while epidemics in small areas (such as city, town) and of other infectious diseases have received insufficient attention.11,14,15 Several methods for detecting and forecasting epidemics, such as time series analysis, were applied. Some methods require strict assumptions in their underlying models and are too complicated to be applied to the data for small areas.13,15 We think that simple methods for detecting epidemics would be more practical for public health activities.
In this study, the possibility of detecting epidemics in their early stage in small areas was evaluated for 16 infectious diseases using a simple method based on data from infectious disease surveillance in Japan in 19931997.
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Materials and Methods |
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Epidemic periods in health centre areas were determined according to their indices. We assumed that an epidemic began in the week when its index exceeded a certain value (the critical value for the onset of an epidemic) and that the epidemic ended the week before indices in four successive weeks were lower than another given value (the critical value for the end of an epidemic). The critical values for the onset of an epidemic of 16 diseases were given between 95 and 99 percentiles of the distribution of indices. The critical values for the end of an epidemic were given as 90 percentiles of the distribution. The critical values for the onset and end of an epidemic were modified if the number of epidemics greatly exceeded or undershot 1200 (1.8 per health centre in 5 years).
Table 1 shows the percentiles of the distribution of indices, the critical values for the onset and end of an epidemic, and the number and mean length of epidemics. The number of epidemics of 16 diseases was between 800 and 1600. The mean length of epidemics was between 2 and 22 weeks.
Method for detecting early stage of epidemic
We used a simple method for detecting the early stage of an epidemic, which is that if the number of cases per week per SMI in a health centre area exceeds a critical value (critical value for an epidemic warning), an epidemic will begin in that area in the following 4 weeks (a warning for an epidemic).
This method is illustrated using a hypothetical example in Figure 1. Consider a warning for an epidemic of influenza-like illness in a health centre area. The index exceeds the critical value for the onset of an epidemic (given as 40) in the 12th week, and is lower than the critical value for the end of an epidemic (given as 10) from the 19th week. Thus, the epidemic period is between the 12th and 18th weeks. The pre-epidemic period (defined as the 4 weeks before the epidemic) is the 8th to 11th week. Non-epidemic periods are between the 1st and 7th weeks and between the 19th and 52nd weeks. Excluding the epidemic period, the indices in the 11th and 40th weeks exceed the critical value for an epidemic warning (given as 10). As the 11th week is in the pre-epidemic period, this warning is a true positive. Inversely, the warning in the 40th week during the non-epidemic period is a false positive.
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Results |
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Discussion |
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Warnings 3 and 4 weeks before an epidemic might be useful for control of the epidemic. Such warnings accounted for between 16.0% and 68.6% of the true warnings in the above ten diseases. Even if a warning 1 or 2 weeks before an epidemic in an area is too late for control of the epidemic in that area, it might be useful for surrounding areas, because epidemics of many infectious diseases generally begin in small areas and subsequently spread widely. This fact is shown using an example of influenza-like illness in Table 4. Thus, the warnings for epidemics in small areas might provide information on epidemics in large areas.
The usefulness of epidemic warnings is related to the possibility of taking countermeasures against the epidemic. Vaccination is an efficient countermeasure for the prevention and control of epidemics of influenza-like illnesses and rubella,4,16 and for these diseases the specificity of the warnings was more than 95% and the sensitivity was more than 80%. In planning prevention and control of an epidemic, characterizing the aetiological agents, such as the type of virus, is as important as evaluating the frequency of cases.6,10 The epidemic warnings presented herein are based only on the frequency of cases. Information on characterizing the aetiological agents is obtained from laboratory-based surveillance.
There were some problems and limitations in the present study. A key problem was the definition of an epidemic, which was done by several methods and not standardized.2,5 The determination of an epidemic in the present study, described above, was according to the number of cases per week per SMI. The critical values for the onset and end of an epidemic might vary by the kind of disease, season, area and so on. In particular, we think that the definition for an epidemic warning should be based on the need for a public health response. We assumed that the pre-epidemic period was the 4 weeks before the epidemic. A pre-epidemic period of over 4 weeks could be discussed. For detecting epidemics in their early stage, the method used requires a stable surveillance system as well as other methods including time series analysis.13,15 We chose the simple method because it would be more practical for public health activities. The sensitivity, specificity and positive predictive value of epidemic warnings might be improved by using more complex methods. The epidemic warnings in the present study were on a health centre area basis because the health centre is the basic unit in the public health activities, including surveillance, against the infectious diseases in Japan. Our results might include potential biases because sentinel medical institutions were recruited on a voluntary basis. For applying the proposed method of epidemic warning to surveillance, further discussion of disease-specific issues would be important.
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Acknowledgments |
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References |
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