Biting back against dengue - Why experts are cautious despite Singapore's success in lowering case count
Years of anti-mosquito efforts have brought dengue infection levels in Singapore to the lowest in recent history. Experts are optimistic but cautioned about popping the champagne too early given perennial issues and emerging risks.
A combination of sustained interventions has brought down the number of dengue infections here, but the disease is hard to eradicate given Singapore's geographical location and urban setting. (Illustration: CNA/Clara Ho)
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Close to a decade ago, Singapore embarked on a project that sounded more like the logline of a science-fiction movie than a national public health plan.
To battle the onslaught of bloodthirsty female Aedes aegypti mosquitoes, that appeared likely to take the annual dengue infections to a record high of 30,000, the National Environment Agency (NEA) built its own army of specially bred mosquitoes.
Created in a facility, these male mosquitoes carried a natural bacterium that meant that any eggs laid by female Aedes mosquitoes they mated with would end up as duds.
Singapore was not the first country to trial this method, but the first to do so in a high-rise, high-density tropical environment.
Now, 10 years on, Project Wolbachia has become a key driver of Singapore's success in its fight against dengue.
"Wolbachia has been a game changer due to its consistent ability to suppress disease-carrying mosquitoes," said Assistant Professor Lim Jue Tao from the Lee Kong Chian School of Medicine at Nanyang Technological University (NTU).
Combined with heightened surveillance, educational campaigns and enforcement efforts over the years, this endeavour has clearly borne fruit.
With reported infections, up to May 9, hitting only 593 – around 35 per cent of the 1,692 cases during the same period last year – Singapore looks on track to record its lowest annual tally of dengue cases in over two decades, assuming numbers grow at their current levels.
The whole of 2025 saw 4,036 cases, which was already a 70.4 per cent drop from the 13,651 cases in 2024. But a quarter century ago in 2001, there were just 2,372 infections.
While infectious disease experts told CNA TODAY they're optimistic that Singapore could hit a record for the lowest number of infections in over two decades, they also cautioned that in the past infections have surged following periods when numbers were relatively low.
Singapore is, in fact, just entering the traditional peak season for dengue from May to October, when the weather is usually warmer and wetter, and more conducive for the breeding of the Aedes aegypti mosquito.
At the start of 2022, for example, weekly cases were in the range of 200 to 300 but soared to over 1,500 in May.
Historical data also shows that one good year does not guarantee the next: From a low 2,767 cases in 2017, infections saw an uptick in 2018 before surging to over 16,007 in 2019.
So has Singapore truly gotten a handle on the fight against dengue, or are mosquitoes likely to make a strong comeback? And what remains to be done before the Republic can truly eradicate this disease – if at all?
KEEPING THESE RAVENOUS PESTS AT BAY
Experts attributed Singapore's success so far to the multi-pronged effort that the country has undertaken for decades.
Dr Asok Kurup, from specialist clinic Infectious Diseases Care, said the numbers this year can be attributed to a combination of sustained interventions including intensified surveillance using Gravitraps – black cylinders placed by NEA all over Singapore that lure and trap female mosquitoes – improved vector control with the Project Wolbachia expansion, and community vigilance.
"I don't think that we are in a 'calm before the storm' scenario because the proactive control measures including surveillance and vigilance have been sustained," he said.
Singapore's main strategy against dengue is vector control, which means suppressing the population of the female Aedes aegypti mosquito, easily identified by their striped legs.
HOW DENGUE SPREADS
When a female Aedes aegypti mosquito bites a person infected with dengue, it becomes a carrier of the virus.
Only female mosquitoes bite as they require protein from blood to develop their eggs. They also tend to be day-biters with peak activity occurring a few hours after sunrise and in the late afternoon before sunset.
However in well-lit environments like in Singapore, these mosquitoes are also known have adapted to artificial lighting and bite at night.
After the virus incubates for about a week, it is transmitted to healthy humans that the mosquito bites subsequently, when the insect injects its saliva into the humans whose blood it's feeding on.
There are four serotypes of dengue viruses: DEN-1, DEN-2, DEN-3 and DEN-4.
Being infected by one serotype provides a person lifelong immunity against that particular serotype, but only a temporary protection of a few months against the remaining three serotypes.
However, after that temporary protection wears off, subsequent infections due to any of the other serotypes can be more severe.
Countries such as Australia, Brazil, Indonesia and Vietnam had already released Wolbachia-carrying mosquitoes, with no negative impact on public health and ecology, noted an expert panel in 2014 supporting the use of Wolbachia technology in Singapore.
"The approach with Wolbachia-carrying Aedes males will likely be most effective when used in combination with other methods of control such as the current community-based removal of potential breeding habitats, and a vaccine when available," Professor Duane Gubler, chairman of the dengue expert advisory panel, had said then.
After further research and lab studies, Project Wolbachia was launched as a small-scale study in parts of Braddell Heights, Tampines West and Nee Soon East in late 2016.
Male Wolbachia-carrying mosquitoes are released twice a week at chosen sites. As they have short lifespans and die usually around a week after release, new mosquitoes need to be released weekly.
With female mosquitoes mating with Wolbachia-carrying males and producing eggs that do not hatch, the mosquito population in an area significantly reduces over time, helping prevent the spread of dengue and a hot spot from forming.
NEA said that it takes three to six months of releases to achieve a significant reduction in the mosquito population.
At sites where Project Wolbachia is carried out, the Aedes aegypti population has been reduced by 80 to 90 per cent, in turn lowering the risk of contracting dengue for residents living in these areas by over 70 per cent.
Even those living in areas adjacent to release sites had their risk of contracting the disease reduced by 45 per cent.
The success of the project has been recognised so widely that Members of Parliament have asked in parliament when their wards will benefit from this project. By the end of this year, the project will expand to cover about half the households in Singapore, or some 800,000.
To support such an expansion, however, Singapore needs a ton of mosquitoes.
NEA expanded its production capacity significantly in 2019 when it opened a S$5 million (US$3.9 million) facility at Ang Mo Kio which could breed five million mosquitoes a week – 10-fold what its original site at Neythal Road could manage.
A second facility managed by Google's subsidiary Debug produces 10 million such mosquitoes weekly after a recent expansion, up from its initial capacity of six million weekly.
The Wolbachia method has mainly been a long-term mosquito population suppression strategy, given that it takes months to create an impact, and thus utilised at areas with high mosquito populations but not active dengue clusters.
However, last year, the agency also began trialling the release of Wolbachia-carrying mosquitoes in active dengue clusters at Sin Ming Road and Jurong West to supplement traditional control operations.
Though the Wolbachia approach has shown positive results so far, both experts and the authorities stressed that it is not a silver bullet, and that it works best when accompanied by other vector control methods.
Indeed, NEA has been carrying out an integrated dengue surveillance and control programme since the 2005 outbreak which saw 14,209 cases, with efforts stepped up periodically during peak seasons and in the face of dengue outbreaks.
For instance in that year, NEA quadrupled manpower for dengue operations to about 500 personnel, while stepping up its inspection frequency to 59,000 a month, up from 55,000 monthly in 2004.
In subsequent years with high case numbers, NEA has worked with town councils and partner agencies to intensify inspections as well as conduct drain flushing and chemical treatments in areas with dengue clusters.
Chemical methods such as fogging are, however, used judiciously due to the negative impact they may have on the environment as well as the potential insecticide resistance that the mosquitos can develop if the method is used indiscriminately.
While Gravitraps do not actually control the mosquito population, they have served as a way to provide surveillance data on population numbers.
From 3,000 units when it was piloted in 2013 pilot, the programme has since expanded to 70,000 Gravitraps across the island.
Enforcement action is also taken against errant companies and households that are found to have mosquito breeding sites.
Prior to 2016, only homes in active dengue clusters found breeding mosquitoes would be fined, but this penalty was extended to all households even outside dengue clusters from March that year. Since 2020, stiffer penalties have also been meted out to households and construction sites where mosquito breeding is found.
First-time offending households now face a S$300 fine if multiple breeding habitats are found in their homes – up from a previous fine of S$200 regardless of the number of such habitats detected. For construction sites, the penalty has been increased to S$3,000 from S$2,000. Recalcitrant offenders may also be jailed.
Authorities have noted, however, that most mosquito breeding spots are found in residential areas.
Clogged gutters, pails collecting rainwater and vases with water that has not be changed frequently are among the usual breeding spots found in homes.
In December 2023, a woman was fined S$1,400 after losing a trial in court for breeding mosquitoes in a toilet bowl that was left unused for prolonged periods. The woman had tried to shirk responsibilities over her action and told officers inspecting her home that the insects "just like fly in to breed there".
This woman's actions are fairly commonplace, given that between 11,000 and 16,000 enforcement actions were taken against premises owners cases each year from 2020 to 2022.
Meanwhile, vaccinations have played a relatively small part in Singapore's fight against dengue.
In October 2016, the authorities here approved Dengvaxia, the first vaccine available against all four serotypes of the disease.
However, deployment was limited to individuals aged 12 to 45 who have a prior documented history of being infected by the disease. Those without previous dengue infection who take the vaccine may be at an increased risk of having severe dengue, if they eventually get infected with the dengue virus.
A second vaccine, Qdenga, was submitted for approval here but its manufacturer later withdrew the application.
In February this year, Health Minister Ong Ye Kung said in parliament that Dengvaxia's supplier has decided to discontinue the vaccine "due to low global demand". He had also previously stated that Singapore does not track the take-up rate of this vaccine here.
Currently, another dengue vaccine candidate, V181, is undergoing Phase 3 clinical trial across multiple countries, which is a large-scale trial to confirm efficacy and effectiveness before applying for regulatory approval.
WHY THERE CAN NEVER BE ZERO DENGUE CASES
Though falling dengue numbers suggest that years of preventive and suppression efforts are paying off, experts warned against complacency, noting that the battle is far from over.
Data from the last two decades have shown several occasions where surges in case numbers followed periods of fewer infections.
For instance, 2012 saw 4,632 dengue cases recorded. The following year, it jumped nearly five-fold to 22,170. Cases remained relatively high until 2016, recording over 11,000 annually during that period.
More recently, 2019 saw dengue infections surge to 16,007, from 3,282 in 2018. And in 2024, cases again jumped to 13,651 from 9,950 the year before.
Several key factors contributed to these sporadic surges, such as the weather, low community immunity, our built-up environment and the change in dominant serotype.
Group director of the Environment Health Institute at NEA, Associate Professor Ng Lee Ching, told CNA TODAY that Singapore's environment is "highly conducive to dengue outbreaks" given the tropical climate and how the Aedes aegypti mosquitoes thrive in man-made environments, breeding in artificial containers or building structures that collect water.
In recent years, the authorities have pointed to warmer and wetter weather as a contributing factor to the rising number of dengue cases.
The warmer temperatures lead to faster breeding of Aedes mosquitoes and shorter incubation period for the dengue virus in them.
Ultimately, Singapore's geographical location and urban setting also make it near impossible to eradicate these vectors entirely, the experts noted.
"The reality is that we live in the tropics and it is highly unlikely that we will be able to eliminate mosquitoes," said Dr Paul Tambyah, immediate past president of the Asia Pacific Society of Clinical Microbiology and Infection.
Experts also pointed out that after decades of successfully suppressing the Aedes mosquito population and the related dengue infection numbers, Singapore residents have a low community immunity to the disease, making them susceptible to occasional outbreaks.
Furthermore, dengue fever is caused by four distinct serotypes. A person who has been infected by one serotype would be immune to it for life, but is only temporarily protected against the remaining three serotypes.
When one serotype that has long been dormant becomes more common, this would lead to a surge as the community would have lower immunity against that erstwhile dormant serotype.
"Outbreak spikes typically occur when high mosquito populations coincide with low immunity in the community and the emergence of highly transmissible virus variants," said Assoc Prof Ng of NEA.
"Consequently, outbreaks are often triggered by a switch in the predominant serotype, signalling lowered population immunity to that specific strain."
This was the case in 2022, when the previously uncommon DENV-3 serotype started circulating, causing an uptick in dengue cases.
THE LIMITATIONS OF PROJECT WOLBACHIA
Ultimately, Singapore, like the World Health Organisation, aspires towards zero dengue deaths.
"Though complete elimination of dengue is unlikely to be feasible, lowering the number of infections will lower the risk of dengue fatalities," Assoc Prof Ng of NEA said.
Despite the progress that has been made in battling a perennial scourge, Singapore remains vulnerable to several risk factors relating to dengue.
For a start, the Wolbachia strategy that has been widely credited for contributing to the decline of the Aedes mosquito population is not without its limitations.
"The technology is expensive, owing to the need for repeated releases of male Wolbachia-Aedes mosquitoes, and needs care in terms of sex-sortings, to prevent inadvertent release of female Wolbachia-Aedes mosquitoes,” said NTU's Asst Prof Lim.
However, the assistant professor in infectious disease modelling added that this technique is "demonstrated scientifically to be cost-effective in the Singapore context".
No data is publicly available on the cost of the Wolbachia project but one study estimated it to be around S$40 million annually.
Then, there is also the question of capacity.
In a parliamentary reply in January on the constraints preventing a speedier roll out of Wolbachia across Singapore, senior parliamentary secretary for the Ministry of Sustainability and the Environment Goh Hanyan said that "resources are finite".
The sites for Wolbachia mosquito releases are based on various considerations such as the historical dengue situations and the current Aedes Aegypti mosquito population at the site, she added.
In 2024, NEA announced that a third facility to produce Wolbachia mosquitos was in the pipeline, but it has yet to materialise.
THE DOWNSIDE TO BEING GLOBALLY CONNECTED
Experts and the authorities acknowledged that decades of relatively low infection numbers have rendered the local community to have lower immunity against the disease.
This may lead to a major resurgence in the future, as some other countries have experienced.
"Aedes aegypti was practically eradicated in Central and South America in the 1960s and 1970s resulting in very few dengue cases," said Dr Tambyah.
"Unfortunately, there was a rebound with the introduction of virus from outside the area and it has proved very difficult to eradicate the vector since.
"Cuba had a very bad epidemic last year with a number of deaths and now with the blockade, things are likely to get worse," he added.
Associate Professor Hannah Clapham, from the National University of Singapore (NUS) Saw Swee Hock School of Public Health, said that while there was lower community against the disease, this is a sign of Singapore's "success story".
However, she cautioned that this would mean that "the amount of effort needed to control dengue increases over time".
In response to queries by CNA TODAY, NEA's Assoc Prof Ng said that the "dengue paradox" – or balancing immediate suppression with the long-term risk of low population immunity – is a complex challenge, as the population remains vulnerable to outbreaks even with a small surge in mosquitoes.
But she said that Singapore's integrated control efforts, together with community vigilance, have kept severe dengue cases low and prevented large outbreaks of Zika and chikungunya, which are diseases transmitted by the same mosquitoes.
"This thus remains the right approach: we protect our population by keeping mosquito numbers low rather than allowing infections to build natural immunity," she said.
Dr Leong Hoe Nam of Rophi Clinic said that while Singapore residents are "relatively safe" in the country, given the low risk of the disease here, they are practically "naked" in terms of protection when travelling out to places where the disease is more prevalent.
This relates to another risk factor that the experts highlighted: the global connectedness of Singaporeans and Singapore as a travel hub.
With thousands of visitors passing through, there is always a risk of individuals bringing in diseases, dengue included.
Dr Tambyah noted that Singapore is actively involved in surveillance efforts in the region to detect new strains of dengue.
"This would help us to be alert to new strains emerging in the region which could be introduced by the hundreds of thousands of visitors who pass through Singapore every day," he added.
GETTING THE COMMUNITY TO CONTINUE FIGHTING THE GOOD FIGHT
Singapore's healthcare landscape has also transformed over the past two decades, with the needs of an ageing population creating new challenges.
Dr Tambyah said that many older Southeast Asian adults may have immunity to all four strains of dengue due to repeated infections across their lifetime.
However, he and other experts also noted that the elderly may suffer from more serious medical complications if infected due to co-morbidities.
There were 52 local dengue deaths from 2021 and 2025.
"The majority of the deaths involved either individuals with pre-existing medical conditions or individuals aged 65 and above. No dengue deaths were recorded among children," Associate Professor Lim Poh Lian, group director of communicable diseases programmes at Communicable Diseases Agency (CDA), told CNA TODAY.
Dr Kurup from Infectious Diseases Care added: "There is a need to make sure that we reach out to our primary care providers to have a heightened index of suspicion of dengue (having lower threshold to diagnose dengue) in the elderly should such a scenario play out."
NUS' Assoc Prof Clapham said that there's ongoing research on the potential burden of dengue cases on the healthcare system.
"As populations age, it will be important to consider what this does to population transmission risk as well as the overall burden of cases in the population."
Another ongoing challenge lies with the complacency of residents, some of whom are unwilling to play their part to help prevent mosquitoes breeding.
After decades of anti-mosquito campaigns, the number of mosquito breeding habitats detected through the years has remained relatively elevated.
About 18,400 such habitats were found in 2014, which then jumped to 23,400 in 2016. In more recent years, annual breeding spots have ranged from between 16,700 to 23,700.
Noting that these occurred against the backdrop of decades of campaigns, Dr Leong of Rophi Clinic said: "Complacency is still high, despite the threat of a jail sentence."
He added that the lack of civic-mindedness in our culture is "embarrassing", giving the example of how litter is commonly strewn in public places and these could potentially become mosquito breeding sites.
For some residents, even as Singapore is seeing a downward trend in dengue cases, this lack of consideration remains frustrating and troubling.
NUS undergraduate, Ms Aris Lim, for example, knows full well that despite being vigilant at home to ensure there is no stagnant water anywhere, she cannot trust that her neighbours, businesses or construction sites near her are doing the same.
As such, she described herself as "just unlucky" for getting dengue twice, despite "doing everything in our power" to avoid it.
Her first dengue infection in 2020 left her with a high feeling and achy joints, while her second infection in 2022 was "unbearable" and "the worst period" of her life.
After enduring the symptoms for about five days in 2022, she was hospitalised for about two weeks where she was put on an intravenous drip of painkillers.
"When the nurses showed me the menu and asked me to pick my meals, just looking at the pictures of so much food made me puke immediately as I hadn’t eaten a proper meal in almost a week due to my nausea," she recalled.
After her two infections, Ms Lim said that her family became concerned about their elderly grandfather who lives with them, and installed window meshes to minimise the chances of mosquitos flying into their home.
"Now I just pray that I never get bitten ever again because I can’t imagine how much worse it can get from my second dengue experience."
