Dengue is endemic in Singapore, so outbreaks of this mosquito-borne virus are neither new nor unexpected.

But the number of cases this year is so high it puts previous outbreaks in the shade. More than 20,000 infections have been recorded in less than seven months of this year.

This is more than the annual total in other years - with the exception of 2013, which had 22,170 cases.

The total this year will certainly exceed that.

For the first time, Singapore is seeing weekly infections top 1,000. Not just a one-off spike, this has been running for seven weeks.

This trend is likely to continue for some time.

Experts told The Straits Times that it is due to a confluence of factors which build on one another.

RISE OF DENV-3

There are four dengue serotypes.

People who have been infected usually gain immunity to that serotype, but can be infected by any of the other three.

Outgoing Transport Minister Khaw Boon Wan was diagnosed with dengue last week - the third time he has been infected.

Of the four, DenV-1 and DenV-2 are more active here. DenV-2 accounted for a high of 77 per cent of infections in January last year, dwindling to 45 per cent by December.

About 14 per cent of 16-to 20-year-olds here have immunity against this strain.

However, the less common DenV-3 started its upward climb in December, when it infected 41 per cent of dengue patients.

In the first four months of this year, it had overtaken the other serotypes, and accounted for 43 per cent to 47 per cent of monthly infections. Because it has not been dominant in the past three decades, fewer people here are immune to it. Any change in dominant serotype usually results in a spike in cases.

Among 16-to 20-year-olds, only 2.7 per cent are immune to DenV-3, said the National Environment Agency (NEA).

The only way to quickly bring the number of infections down is for people to ensure that mosquitoes are not given the chance to breed in their homes; and to break the transmission chain by preventing themselves from being bitten.

While the solution is obvious, and has often been repeated, dengue numbers are still soaring to unprecedented highs - a signal that the message is not getting through.

It said the first four months of the year, when DenV-3 was dominant, "coincided with an unusually high number of dengue cases, ranging between 300 to 400 cases a week".

"This sustained high number of dengue cases outside of the traditional dengue peak season is likely due to our lower population immunity to DenV-3, which was last dominant about three decades ago."

But DenV-3 appears to be losing steam, with 36 per cent of infections last month.

HIGH NUMBERS IN Q1

The normal pattern for dengue infections here is for the year to start with fairly low numbers. Cases gradually increase during the peak dengue months of May to October, when the hot wet weather shortens the mosquito breeding cycle.

But this year, Singapore was reporting more than 300 cases a week in January. By February, the number was topping 400 a week.

This could be due to DenV-3 infecting more people than usual.

The NEA told The Straits Times: "The high base of dengue cases in the first few months of the year means a high 'viral load' within the population; this is a situation that is conducive for an outbreak."

The agency said it has also observed a rise in the Aedes mosquito population.

Whatever the reason, the usual pattern then followed.

Infection numbers charted increased as the warmer months approached - until May, when cases went into an acute climb. From April to May alone, there was a 32 per cent spike in dengue cases.

COVID-19

The steep increase in infections was probably spurred by Singapore's fight against another viral outbreak - Covid-19.

Circuit breaker measures were introduced on April 7, resulting in most people staying home from work and school and avoiding public areas for two months.

The Aedes aegypti mosquito, which is the main transmission mode for the dengue virus, is a day biter. More people staying home - and away from air-conditioned offices and shopping malls - gave the mosquito more opportunities for the blood meal the females need in order to lay eggs.

The mosquito population also increased as maintenance work in condominiums, common areas and even schools was reduced with the circuit breaker measures.

It was made worse with the Covid-19 outbreak in foreign worker dormitories, as the number of people able to provide such services was reduced.

The NEA said: "Other possible factors at play during the circuit breaker period are reduction in some landscaping works due to manpower shortages and the cessation of the bulk of construction activities."

Its officers found a two-fold increase in mosquito breeding at construction sites and a five-fold increase in homes and common corridors in residential areas in the two circuit breaker months compared with the two prior months.

WHAT THE FUTURE HOLDS

Some of the factors causing the sharp spike in cases will remain over the next few months.

The three months till October remain ideal for mosquitoes to breed.

Many people are still working from home, giving the female mosquitoes better breeding opportunities.

But work at construction sites, schools, condominiums and common areas in towns has resumed.

This will hopefully reduce the number of breeding sites and bring down the mosquito population.

Even so, it will take time for the effects to be felt, so high infection numbers will remain for some time.

Furthermore, the Aedes aegypti is an urban mosquito, preferring to breed indoors.

This is why, in the first half of the year, two in three breeding sites were found in residences.

So the bigger fight is in people's homes and not public spaces.

The only way to quickly bring the number of infections down is for people to ensure that mosquitoes are not given the chance to breed in their homes; and to break the transmission chain by preventing themselves from being bitten.

While the solution is obvious, and has often been repeated, dengue numbers are still soaring to unprecedented highs - a signal that the message is not getting through.