Last week's total of 544 new dengue cases is the highest recorded for the month of January, traditionally the low season for this viral disease. The figure, released yesterday, was 96 up on the previous week, and the third weekly rise.
There have been another 121 cases between Sunday and 3.30pm on Monday this week.
Professor Leo Yee Sin, head of the Institute of Infectious Diseases and Epidemiology at Tan Tock Seng Hospital, called the trend "rather unusual". "The situation requires close monitoring and it is important for everyone to do their part in strengthening the environmental prevention effort," she said.
The National Environment Agency (NEA) said the change of the main strain of virus from Den-1 to Den-2 - there are four strains of the virus - "may be an early indicator of a future dengue outbreak".
In 2014, four in five dengue patients were infected with Den-1. But Den-2 started rising from around August last year and now accounts for two in three infections.
In a joint statement to The Straits Times, the NEA and the Ministry of Health (MOH) said that although Den-2 was the dominant strain from 2007 to 2012, immunity in the population is low.
Number of people down with dengue in Tampines , the biggest of 108 active clusters
A 2013-2014 study found that less than 15 per cent of those aged between 16 and 25 were immune to Den-2.
Last month, the NEA issued nine stop-work orders to construction sites found breeding mosquitoes.
However, none was in Tampines, which now has the biggest cluster with 206 cases, 33 within the past fortnight. There are currently 108 active dengue clusters, mostly in the eastern half of the country.
Last year, more than 2,200 dengue patients were warded in public hospitals out of 11,298 people diagnosed with the disease. An MOH spokesman said this was in line with the norm of one in five dengue sufferers needing hospital care.
A few countries, including the Philippines, have introduced the first available dengue vaccine.
But Prof Leo said it might not be as effective in Singapore as it gives better protection against Den-3 and Den-4, which are not the dominant strains here.
Singapore has a large adult population that has never had dengue. People who have been infected are protected against only that viral strain and not the other three.