A tiny bacterium called Wolbachia is Singapore's stealth weapon against the spread of dengue fever.
When infected male mosquitoes breed with wild females, the resulting eggs do not hatch - bringing down the mosquito population in the long run.
Laboratory tests using Wolbachia started at the National Environment Agency's (NEA) Environmental Health Institute in 2013.
Three years later, Environment and Water Resources Minister Masagos Zulkifli announced in Parliament that a small-scale field study would be carried out to trial the mosquito control technique in the real world.
Later that year, infected Aedes aegypti mosquitoes were released at Braddell Heights, Tampines West and Nee Soon East. Traps were placed in some areas - including the homes of residents - to recapture mosquitoes and determine how far they had flown from the original release point.
Scientists later found 50 per cent fewer adult mosquitoes in those districts, compared with areas where the trial had not taken place.
Mosquito eggs were also collected from these sites and half of them did not hatch.
The first phase of the study had its limitations.
Non-infected male mosquitoes moved easily from surrounding areas to the field study sites, limiting the impact of the Wolbachia-carrying mosquitoes.
And only 6 per cent of Wolbachia-infected mosquitoes were later found on the ninth floor or higher, and only half of the mosquitoes survived to four days.
Female mosquitoes infected with Wolbachia had also slipped through the sorting process, which can pose a problem as they can still go on to have offspring. In the long term, this could impact the ability of male Wolbachia-carrying mosquitoes to suppress the urban mosquito population.
In February this year, NEA announced that it would be embarking on its second phase of Project Wolbachia, with mosquitoes released at 76 Housing Board blocks and on higher floors. They were also released more frequently than before to keep up the population for a longer time.
"A larger reduction in egg hatch and adult population will be necessary to achieve suppression of the urban Aedes mosquito population," said the NEA in a statement that month.
It added that X-ray treatment will be used to render Wolbachia-carrying female mosquitoes infertile.
In September, the agency said it would be tapping technology to mass-produce mosquitoes on a larger scale, employing technology to speed up the labour-intensive process.
This includes counting mosquito larvae, sorting them by gender, and even automated methods of releasing them into the field.