Climate variability: an underestimated driver of the risk of vector-borne disease outbreaks

A study published in PNAS demonstrates that internal climate variability—the natural and chaotic fluctuations in climate that are independent of human emissions and various risk models—is a major source of uncertainty regarding the transmission of vector-borne diseases (dengue, chikungunya, West Nile, etc.). Overlooked in most studies, this variability can create conditions favorable to transmission much earlier than the warming trend alone would suggest. The authors call for this variable to be integrated into the core of health surveillance plans.  

The Blind Spot of Internal Climate Variability

Climate-epidemiological projections typically consider three sources of uncertainty: emissions scenarios, structural differences between climate models, and internal variability. Most analyses focus on the first two. However, this study shows that, over the course of several decades, internal climate variability appears to be a key factor in the spread of infections via mosquitoes—particularly those of the Aedes genus—resulting in earlier and more frequent transmission opportunities, even in areas that are currently non-endemic.  

Impacts on public health

In practice, basing the surveillance of vector-borne diseases solely on the trend of average climate change delays the necessary responses (triggering action plans, deploying resources, informing the public, and making vector control decisions). The PNAS study suggests shifting to proactive plans that account for the wider ranges of uncertainty caused by internal climate variability. In other words, action should be taken earlier and more frequently in years when conditions are favorable for vectors, even if the average climate risk does not yet appear to be critical.  

Operational Implications According to the “One Health” Approach

Surveillance must become adaptive, with sentinel systems (clinical, entomological, virological) configured based on probabilistic thresholds that incorporate internal climate variability (e.g., more intense El Niño effects or local temperature and precipitation anomalies) rather than fixed schedules. Accounting for seasonality must be dynamic, with more frequent elimination of larval breeding sites and control campaigns (adulticides, larvicides) adjusted according to forecasts of climate anomalies. Communication regarding epidemic risk is best supported by the dissemination of awareness messages based on variability indicators (extreme heat or humidity, heavy rains) to accelerate collective mobilization (elimination of breeding sites, individual protection). Prevention plans must incorporate scenarios involving a massive influx of patients into hospitals—even in settings considered non-endemic—to prevent shortages of test kits, the saturation of laboratory facilities, and bed shortages.  

Adapting Prevention Measures

Health agencies and local governments are encouraged to adopt a probabilistic and opportunistic approach, with surveillance and actions that keep pace with climate variability. This involves incorporating internal climate variability into risk modeling and funding specifications, linking contingency plans to climate triggers such as the El Niño and Southern Oscillation (ENSO) index, the North Atlantic Oscillation (NAO), precipitation and temperature anomalies, and to assess the costs and benefits of early interventions—even if the event ultimately does not occur—in accordance with the precautionary principle.   Ultimately, internal climate variability is an underestimated amplifier of vector-borne risk: ignoring it risks reacting too late. Conditions conducive to vector-borne transmission may arise earlier than initially anticipated. Prevention must become flexible, probabilistic, and based on indicators of variability, not solely on seasonal or local factors.