Health early warning systems

Climate patterns and weather events play a role in determining the incidence and geographic range of a wide range of infectious diseases; in principle, public health and health care authorities could use early warning systems to identify where and when additional interventions could reduce disease outbreaks. Early warning systems are also useful for reducing morbidity and mortality during and following heatwaves, floods, windstorms, and other extreme weather events.

The principal components of an early warning system include forecasting when and where additional cases of illness or death are likely to occur, predicting the number of cases, implementing an effective and timely response plan, and ongoing evaluation of the system and its components. The distinction between prediction and early warning is important: early warning is prediction but not all predictions are early warnings. The relative importance of forecasting the timing of an outbreak and predicting the actual number of cases will depend on the disease control decisions that will be taken and the degree of interannual variation of the disease. In some instances, forecasting the timing of a likely outbreak is sufficient. A response plan needs to include detailed implementation plans and evaluation of effectiveness. Implementation plans should, at a minimum, include modules addressing where the response plan will be implemented; when interventions will be implemented, including thresholds for action; what interventions will be implemented; how the response plan will be implemented; and to whom the interventions will be communicated.

The sensitivity and specificity of the predictive model used have implications for the design of interventions. False positives (issuing a warning when none was required) and false negatives (not issuing a warning when one was needed) have consequences, not only in terms of morbidity and mortality, but also in terms of public willingness to rely on subsequent warnings. Incorporating understanding of these uncertainties and their associated costs into the design of an early warning system can improve its effectiveness.

Limitations in long-range forecasting, monitoring of environmental variables, and case surveillance are challenging development of accurate and timely infectious disease early warning systems. Another difficulty lies in linking climate and health data at a suitable high resolution; weather stations are often widely spaced and may not describe either local variation in climate or microclimatic conditions in specific important environments (e.g. resting sites of adult mosquitoes).

Despite the many uncertainties and constraints, recent research suggests significant promise in using climatic and environmental variables to develop early warning systems for a wide range of infectious diseases. Climate change projections suggest increased risks for disease outbreaks with changing weather patterns, making the need for more effective warnings systems imperative.