In February 2025, researchers from Princeton University published a study in GeoHealth revealing that consecutive El Niño events can lead to prolonged and intensified infectious disease outbreaks. The study, titled "Intersecting Memories of Immunity and Climate: Potential Multiyear Impacts of the El Niño–Southern Oscillation on Infectious Disease Spread," highlights the complex interplay between climate variability and population immunity, emphasizing the need to incorporate climate patterns into public health planning.

The El Niño–Southern Oscillation (ENSO) is a periodic fluctuation in sea surface temperatures and atmospheric pressure over the equatorial Pacific Ocean, influencing global weather patterns. El Niño events are characterized by warmer ocean temperatures, while La Niña events feature cooler temperatures. These climatic shifts can affect environmental conditions such as temperature, precipitation, and humidity, which in turn influence the transmission dynamics of various infectious diseases.

The Princeton study utilized a susceptible-infected-recovered-susceptible (SIRS) model to simulate disease dynamics in response to ENSO-induced climatic changes. The researchers found that the climatic changes associated with consecutive El Niño events can disrupt the balance of susceptible and immune individuals in a population, resulting in more severe and extended outbreaks. The study states, "El Niño-Southern Oscillation (ENSO) impacts on infectious disease outbreaks may last multiple years via lagged nonlinear effects on population immunity." This suggests that the effects of ENSO on disease can grow in magnitude and last beyond the duration of ENSO events due to the lasting effects of population immunity on infections.

A pertinent example of ENSO's impact on infectious diseases is the surge in dengue fever cases in Peru during the 2024 El Niño event. In the city of Piura, residents faced water shortages, leading them to store water in containers that became breeding grounds for mosquitoes. The combination of high temperatures and increased mosquito populations resulted in a significant rise in dengue cases. By early March 2024, Piura reported 5,275 cases, making it the second hardest-hit city in Peru. Nationally, over 34,000 cases were recorded in the first eight weeks of the year, doubling the number from the same period in 2023. The Peruvian government declared a health emergency in most provinces, and health officials acknowledged that the situation was not under control, with cases expected to continue rising.

The study's findings have significant implications for public health planning and policy. Public health authorities should incorporate climate variability data, particularly ENSO forecasts, into disease prediction models to enhance the accuracy of outbreak forecasts. Understanding the prolonged effects of ENSO on disease dynamics can inform the development of adaptive response strategies, such as targeted vaccination campaigns and vector control measures, timed to mitigate anticipated outbreaks. Recognizing the potential for extended and intensified outbreaks following consecutive ENSO events can guide resource allocation, ensuring that healthcare systems are adequately prepared to handle increased caseloads.

The Princeton study underscores the critical need for integrating climate data into public health planning. By highlighting the prolonged and amplified effects of consecutive El Niño events on disease outbreaks, it emphasizes the importance of developing effective strategies to predict, prepare for, and mitigate the impacts of climate-induced health crises.