Risk drivers for dengue cases in 2021 include the continued high Aedes aegypti mosquito population and the current moderate number of dengue cases, amid higher circulation of the less common Dengue virus serotypes 3 and 4 across the island. Many residents also continue to work from home, which from the experience in 2020, has been shown to increase dengue risk. 

Singapore, 25 January 2021 – The number of dengue cases has remained relatively stable for the past few weeks, dropping to 156 reported cases last week. This is less than half of the corresponding weekly case number of 343 seen in the historic outbreak year of 2020. The latest weekly number of cases remains moderate, and while lower than 2019, it is higher than this time of the year in 2017 and 2018. The Aedes aegypti mosquito population has also shown an increase of about 8 per cent in the month of December 2020, and remains high in some areas of Singapore. Of significance is also the higher proportion of cases of the less common Dengue virus serotypes 3 and 4 (DENV-3 and DENV-4) which has been detected across the island, and together make up more than half of the serotypes sampled. If left unchecked, the current high Aedes aegypti mosquito population, coupled with the not insignificant number of dengue cases currently and the sizeable proportion of residents still working from home, would add to the dengue risk this year. NEA therefore urges all members of the public and stakeholders to stay vigilant, and work together as a community to stem dengue transmission.

Review of Dengue Situation in 2020

2          Due to a confluence of factors, some COVID-19 related, 2020 was a historic dengue outbreak year for Singapore, with 35,315 dengue cases reported. The year started with a high of 303 weekly cases, and the relative dominance of the less common Dengue virus serotype 3 (DENV-3) in the first four months of the year. The number of weekly cases rose sharply in May 2020, shortly after the start of the Circuit Breaker period, and eventually peaked in July 2020. In response, our community stakeholders rallied together to fight dengue, despite the exceptional challenges posed by the COVID-19 pandemic and the unprecedented high number of dengue cases.

3          NEA, together with its partners from the Inter-Agency Dengue Task Force (IADTF) and the Town Councils, maintained a high tempo of vector control efforts throughout 2020, by conducting intensified checks and sustained outreach to remove mosquito breeding habitats, and stepping up chemical treatment interventions to kill adult mosquitoes. Town Councils worked with NEA to conduct an intensive two-week islandwide exercise in July 2020 to fight dengue, while various other measures were undertaken by IADTF members to ensure that their premises were well maintained. The Singapore Contractors Association Ltd (SCAL) intensified audits of its member sites, especially when Circuit Breaker measures were eased, and the construction workforce gradually returned to the worksites. The Singapore Pest Management Association (SPMA) also raised awareness through their pest control members, and maintained high levels of vigilance on the ground. Other efforts included residents checking their homes frequently for potential mosquito breeding habitats, and using insect repellent and insecticide, to protect themselves and their loved ones and to help to break the dengue transmission chain. Stakeholders were mobilised to work closely with Grassroots Leaders and community volunteers, to reach out to residents on dengue prevention and control measures. Together, these efforts have been critical in bringing down dengue transmission from August 2020 onwards, and finally to below 300 in the third week of November and below 200 in the fourth week of December.

4          The present weekly case number above 160 is moderate for this time of the year. It is lower than the average weekly case number in 2019 and 2020, but higher than that in 2017 and 2018, for this time of the year. We therefore urge everyone to work with NEA, and do their part to remove stagnant water from our environment, to deprive the mosquitoes of their breeding habitats and help break the dengue transmission chain.

5          In 2020, NEA conducted about one million inspections for mosquito breeding islandwide, including about 7,400 checks at construction sites, and uncovered about 23,400 mosquito breeding habitats. During the same period, 8,100 enforcement actions [1] have been taken against owners of premises for mosquito breeding. About 430 summonses and 37 Stop Work Orders were issued to construction sites, and 27 contractors were charged in court for repeat offences. Since implementation of the heavier penalties against homes on 15 July 2020, NEA has detected multiple mosquito breeding in 346 of the residential premises inspected, and repeated mosquito breeding in about 330 of the residential premises inspected.

Possible Rise in Less Common Dengue Serotypes in 2021

6          The predominant Dengue virus serotype in Singapore has remained as Dengue virus serotype 2 (DENV-2) since 2016. However, we have seen an increase in the proportion of  Dengue virus serotypes 3 and 4 (DENV-3 and DENV-4) cases and clusters over the past month. As Singapore has not seen a DENV-3 outbreak in the three decades before 2020, and has not seen a large recorded outbreak of DENV-4, the population immunity for DENV-3 and DENV-4 is low, and more people are susceptible to transmission of the virus. This could result in a continued high risk of dengue transmission given the current moderate number of dengue cases and the relatively high Aedes aegypti mosquito population. It is thus critical that all residents and stakeholders work closely together with NEA to break dengue transmission in these clusters, and curtail the spread of the virus. As of 22 January 2021, amongst the five largest dengue clusters, DENV-3 was detected in the cluster at Jalan Gembira / Jalan Mesra, and DENV-4 was detected in the clusters at Bedok North Street 3 and Gangsa Road.

7          NEA will continue to adopt a high tempo of preventive inspections to remove mosquito breeding habitats, and further reduce the risk of dengue transmission. Since November 2020, despite the improved dengue situation and start of the usual year-end lull, NEA has also been conducting operations on the ground, in areas with high Aedes aegypti mosquito population, which are adjacent to dengue clusters or have not experienced dengue transmission in 2020. Construction sites will also continue to be areas of focus. To safeguard public health, NEA will not hesitate to take action against residents and contractors for any mosquito breeding found in their premises. In particular, NEA will take strong enforcement action against recalcitrant errant owners of premises.

Figure 1. Number of dengue cases from 2017 to 2021

Advisory for Chinese New Year Period

8           Aedes mosquitoes can breed in clean stagnant water found in homes and other premises. As the Chinese New Year festive season approaches, homes and other premises are decorated with more ornamental plants. In anticipation of this, NEA has completed inspections of all plant nurseries, and has distributed advisories and pamphlets to all nursery operators, reminding them to maintain vigilance and prevent mosquito breeding ahead of the Chinese New Year period. All stakeholders are urged to prevent mosquito breeding and keep the mosquito population low, by immediately and regularly practising the following steps to remove stagnant water:

• Turn the pail
• Tip the vase
• Flip the flowerpot plate
• Loosen the hardened soil
• Clear the roof gutter and place Bti insecticide inside

9         Homeowners doing spring cleaning are also reminded to properly dispose of any refuse, including large furniture or household items, to avoid the discarded materials from becoming unintentional mosquito breeding habitats. Those decorating their homes and premises with ornamental plants are urged to take care of these plants and prevent them from becoming mosquito breeding habitats, by making sure that water does not accumulate in the flowerpot plates or on top of the hardened soil.

10        As there is a higher tendency for water to remain stagnant in unused premises, those planning to go on staycation spanning a few days should take the following extra steps to prevent mosquito breeding:

• Cover all toilet bowls in your home, and seal off overflow pipes of the flushing cisterns
• Cover all floor traps
• Add Bti to places where mosquitoes could potentially breed, and places where stagnant water cannot be removed
• Clear debris and blockages, and place Bti insecticide in roof gutters and drains within compounds
• Turn over all water storage containers and wipe the rims dry
• Ensure that flower pots, plates and trays do not collect water, after watering your plants
• Ask a relative or close friend to check your home regularly for stagnant water, if you will not be around for a long period of time
• Leave your contact details with your neighbours or the neighbourhood police post/ centre, so that you can be easily reached if needed