Should Singapore bring in vaccine?

One dengue-protection measure that has not yet been implemented here is vaccination.

Professor Annelies Wilder-Smith, from Nanyang Technological University's Lee Kong Chian School of Medicine, said the Dengvaxia vaccine that has been approved in Mexico, Brazil, El Salvador and the Philippines may not be that effective in Singapore, as it works better in areas where more people have already been infected. This is because the immune system of a previously exposed person "remembers" the virus and responds more vigorously the next time, and the vaccine helps to boost that response.

The difficulty of making an effective dengue vaccine is compounded by the need to simultaneously protect against multiple strains of the virus, and more research is needed, said Dr Leong Hoe Nam, infectious disease specialist at Mount Elizabeth Novena Hospital. An infected person acquires immunity against only the strain that infected him or her, and subsequent or secondary infection by another strain often leads to much more serious illness.

Prof Leo Yee Sin, director of the Institute of Infectious Diseases and Epidemiology at Tan Tock Seng Hospital, noted that most of the seniors who have died of dengue in Singapore had secondary infections.

There are five strains of dengue, three of which have been active here. The fifth strain was discovered in a 2007 outbreak in Sarawak and positively identified only in 2013. That strain is so far confined to Sarawak. Dengvaxia protects against all but this strain. The vaccine is not currently approved by the World Health Organisation (WHO) to be administered to people aged below nine or above 45.

Duke-NUS Medical School's Prof Duane Gubler said the duration of immunity conferred by the new vaccine, as well as its ability to give continued protection to people as they enter old age, is not yet known and will take years to determine.

Nevertheless, WHO's former director of research policy & cooperation Tikki Pang thinks Singapore should bring in the new vaccine as soon as possible.

The Health Sciences Authority is currently evaluating the vaccine. Prof Wilder-Smith said epidemiologists and modellers are estimating the effects of the vaccine in Singapore, and a decision on whether to introduce the vaccine here is expected by October this year. Meanwhile, at least four other dengue vaccines are in various stages of development.

Lin Yangchen

A version of this article appeared in the print edition of The Straits Times on June 02, 2016, with the headline 'Should Singapore bring in vaccine?'. Print Edition | Subscribe