Low immunity among the population against the dengue virus may be contributing to a surge in infections this year, especially as mosquito numbers remain persistently high, said health and environment experts yesterday.
This comes as dengue cases spike in the region, indicating that the Aedes mosquito - the carrier of the disease - may be thriving with a warmer weather pattern this year.
At a media sharing session in commemoration of Asean Dengue Day 2019 a week ago, a panel of experts also raised concerns about patients over 65, who tend to suffer more, yet typically do not display the usual dengue symptoms.
These patients are likely to have contributed to the small uptick in the average hospitalisation rate for the disease in recent years.
Singapore is in the midst of a dengue outbreak, with five reported deaths from the disease so far this year. As of last Monday, there were 5,261 infections reported, a 60 per cent increase over the total for the whole of last year, and close to double of 2017's overall numbers.
Associate Professor Ng Lee Ching, director of National Environment Agency's (NEA) Environmental Health Institute, said Singapore has low herd immunity against the disease as good mosquito control over the years means that many people have never been infected.
An NEA study on the prevalence of the disease in healthy adults showed that only about 16 per cent of Singapore's youth aged between 16 and 20 years in 2009 had had dengue before, she said.
Number of dengue infections reported this year as of last Monday, a 60 per cent surge over that for the whole of last year.
The study, which is the latest one available on the topic, was published in 2015.
A recovered patient is immune to the particular strain of dengue he or she was infected with.
"When we say we have low herd immunity and that we are very sensitive to outbreaks, that means that we just need a few mosquitoes to have an outbreak," said Prof Ng.
Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases, said a particular group of concern was the elderly.
A study published last month that reviewed dengue hospitalisations showed a higher proportion of dengue patients hospitalised from 2003 to 2017 were the elderly. In 2017, 47.2 per cent of dengue patients admitted to hospitals were aged 65 and above.
This has probably contributed to the slight increase in the average hospitalisation rate for dengue, from 2014's low of 25.6 per cent, to 2017's 35.9 per cent, said Prof Leo.
The elderly tend to face more complications with the disease because they may have existing chronic conditions, and they are more susceptible to secondary infections, she said.
Another factor which compounds the problem is the difficulty of diagnosing dengue in the elderly, as they tend not to display the typical symptoms such as fever, body aches and rash, according to experts.
Dr Wong Wei Mon, a senior physician with the College of Family Physicians Singapore, said the elderly may have only fever or nausea when they visit the doctor, or may even have symptoms not listed as common dengue symptoms.
"Doctors, if they are not alert enough, may not trigger the definitive test to confirm the presence of dengue," said Dr Wong. By the time they are diagnosed, the disease may be severe, he added.
To combat this, the authorities have been urging doctors to keep a keen eye out for dengue among the elderly, said the experts.
The NEA, too, has been ramping up efforts to prevent mosquito breeding, such as introducing Aedes moquitoes infected with the Wolbachia bacteria into the wild to help reduce the mosquito population, said Dr Christina Liew, NEA's medical entomologist.