Early warning systems
Early warning systems are in most instances, timely surveillance systems that collect information on epidemic prone diseases in order to trigger prompt public health interventions. However, these systems rarely apply statistical methods to detect changes in trends, or sentinel events that would require such intervention. In most cases they rely on an in-depth review done by epidemiologists of the data coming in, which is rarely done in a systematic way.
Such systems require a switch of paradigm. Statistics are not used to detect and confirm an epidemic requiring action, but should used as a way of prioritizing review of data on a regular basis. This process should be guided by statistical tests that will allow the epidemiologist to focus on data cells (defined by time, place and disease) where a significant change has occurred, and forget cells where nothing significant has happened. In other words, statistics should be used to extract significant changes drowned in routine tables of weekly data. To allow this activity specific indicators and thresholds need to be developed to alert for action. Thresholds can be expressed:
- In absolute number (e.g.1 case of polio, or one case of rabies) for sentinel events or point source outbreaks.
- In relative values, starting from historical data expressed either in magnitude of the increase (the most recent 4 weeks average, compared to similar 4 week periods averaged over the past 3 years).
- An absolute rate over the past 52 weeks (such as more than 2 cases /100 000 population for meningococcal meningitis).
- Statistical cut-offs: departure from historical data can be expressed taking into account the time dependency observed in time series by statistical techniques, such an increase above two standard deviation of historical values ( This requires time series modelling that uses a regression approach).
WHO is strengthening existing surveillance systems for infectious diseases, developing early warning systems based on the above concepts and techniques. Countries where an early warning system has been implemented in 2002 are Egypt, Iraq, Jordan and Serbia.