SINGAPORE - New strains of less common dengue serotypes, which are usually imported, could raise the risk of a larger outbreak, say experts.

Of the four distinct dengue virus variations, the DenV-1 and DenV-2 serotypes are most commonly seen here. The majority of infections since 2016 have been caused by DenV-2 - meaning that most recovered dengue patients would have immunity against this serotype.

However, DenV-3 and DenV-4 accounted for more than half of the dengue cases sampled here since February.

Last year saw the first DenV-3 outbreak here in three decades and a fourfold spike in DenV-4 cases, from 4.8 per cent of infections in January to 23.1 per cent by December.

At its peak, DenV-3 accounted for 47 per cent of last year's infections. In contrast, this serotype made up just 11 per cent of the cases between 2014 and 2018.

A virus serotype also has distinct strains, which are genetically different but create the same immune response to different degrees, depending on the strain, in the body.

More than 2,700 dengue cases have been reported since the start of the year, which is fewer compared with the same period last year.

There were 35,315 reported dengue cases last year, when there was a major outbreak.

The National Environment Agency (NEA) told The Straits Times that new strains are usually introduced through imported cases. Such viral strains can subsequently circulate more widely due to factors such as lower population immunity to the less common serotypes and better virus replication in the mosquito and human.

Warmer and rainier weather can also lead to rising mosquito populations and increased feeding.

Professor Paul Tambyah, president of the Asia-Pacific Society of Clinical Microbiology and Infection, said it is possible for one strain to become more dominant if immunity to the other strains have developed.

He noted that past studies by the NEA found that several DenV-3 strains had been behind infections in different areas in Singapore.

Analysis of those DenV-3 samples had identified three genotypes closely related to those found in Indonesia, Malaysia, the Philippines, Thailand, Saudi Arabia and Cote d'Ivoire.

The NEA said the most common DenV-3 strains had spread in Singapore in 2015 and 2017 and are closely related to the strains found in South-east Asian countries.

With the drastic reduction in international travel this year, however, it is less likely that a new "variant" of DenV-3 would have been introduced, Prof Tambyah said.

Nonetheless, noting that the Delta variant of the Sars-CoV-2 virus has managed to seep in despite the curbs in international travel, it is still possible for DenV-3 strains to be brought in by travellers, he added.

Professor Lok Shee Mei from the Emerging Infectious Diseases Programme at Duke-NUS said mosquitoes infected with DenV-3 could possibly have been brought into Singapore via freight, particularly from countries in Asia, South-east Asia and from Australia where the serotype has been endemic.

The recent rise in dengue cases could be due to more people working from home during the period of heightened alert. This gives the Aedes aegypti mosquitoes, which are day biters, more opportunities and some of them may carry the DenV-3 serotype.

This was also the postulation for the 50 per cent spike in dengue infections during the circuit breaker period last year.

Assistant Professor Vincent Pang, director of the Centre for Infectious Disease Epidemiology and Research at the National University of Singapore's Saw Swee Hock School of Public Health, said the risk of getting bitten by DenV-3-infected mosquitoes depends on several factors. These include whether there is mosquito breeding in the area.

DenV-3 infections would also likely be higher in areas where people have low immunity against the serotype, such as when there is herd immunity against both the DenV-1 and DenV-2 serotypes.

"However, if there is sustained compliance to dengue preventative measures, such as removing stagnant water to prevent mosquito breeding and using insect repellent, then the risk of getting bitten would be much lower," he said.