Dengue cases plunge to 7-year low

Key Concepts

• Dengue Fever (Deni): A mosquito-borne viral infection.
• Wolbachia: A naturally occurring bacteria used to control mosquito populations.
• Project Wolbachia: Singapore’s national program utilizing Wolbachia-carrying mosquitoes to suppress the Aedes aegypti mosquito population.
• Aedes aegypti: The primary mosquito vector for dengue transmission.
• Herd Immunity: The indirect protection from infectious disease that occurs when a sufficient percentage of a population has become immune to an infection.

Dramatic Decline in Dengue Cases in Singapore – 2025 & Project Wolbachia’s Impact

Singapore is experiencing a significant decrease in dengue fever (deni) cases. The National Environment Agency (NEA) reported approximately 4,000 cases in 2025, representing a 70% reduction compared to previous years. This marks the lowest number of deni cases recorded in Singapore in seven years. Correspondingly, dengue-related deaths have also fallen sharply, from 17 in 2024 to only four recorded as of 2025.

Project Wolbachia: Methodology and Efficacy

The primary driver behind this decline is attributed to the success of Project Wolbachia. This project involves the systematic release of male Aedes aegypti mosquitoes carrying the Wolbachia bacteria. Wolbachia is a naturally occurring bacterium that prevents the dengue virus from replicating within the mosquito, and crucially, when male Wolbachia-carrying mosquitoes mate with wild female mosquitoes, the eggs are inviable, leading to a reduction in the Aedes aegypti population.

A study demonstrated the effectiveness of this approach, revealing a greater than 70% reduction in the risk of contracting dengue at sites where Wolbachia mosquitoes were released. Adjacent sites also experienced a 45% reduction in dengue risk. Currently, Project Wolbachia covers over 580,000 households across Singapore, indicating a substantial nationwide implementation.

Contributing Factors & Expert Perspectives

While Project Wolbachia is considered the major contributing factor, Professor Pangestu, an infectious diseases expert interviewed by CNA, suggests additional factors may be at play. These include potentially reduced travel and drier weather conditions, both of which can influence mosquito breeding and transmission rates. He stated, “I believe that NEA has been expanding this technology to include more and more households and that could be to me one of the main reasons for the big drop that we've seen in addition to the possibility of less travel and maybe uh drier conditions.”

Concerns Regarding Sustained Immunity & Future Risk

Despite the positive trend, Professor Pangestu cautions against complacency. He highlights the potential for a decline in population immunity to the dengue virus due to the reduced number of infections. Fewer infections mean fewer individuals are developing natural immunity, potentially creating a larger susceptible population. He poses the question: “What does that mean in 2026 and beyond? Does that mean that fewer people in Singapore have immunity to the deni virus simply because there there's fewer infections there and few viruses going around?”

This raises the theoretical possibility that the current positive trend could be reversed in the future – potentially within two to three years – if transmission rates were to increase again. He elaborates, “So I [laughter] mean this is mainly theoretical. If that happens, could the positive trend that we're seeing be reversed? Maybe not next year but the year after or 2 3 years from now because fewer people are immune to the dangi virus.” This underscores the importance of continued monitoring and potentially proactive measures to maintain population immunity.

Synthesis

The significant reduction in dengue cases in Singapore in 2025 is largely attributable to the successful implementation of Project Wolbachia. While other factors may contribute, the data strongly supports the efficacy of releasing Wolbachia-carrying mosquitoes in suppressing the Aedes aegypti population and reducing dengue transmission. However, the potential for decreased population immunity due to fewer infections presents a future challenge that requires ongoing vigilance and strategic planning to ensure the sustained control of dengue fever.