2016: the year of the dengue vaccine

Considered the most important mosquito-borne viral disease, dengue today affects at least 50 to 100 million people every year, with about one per cent or 500,000 of those affected suffering from potentially deadly haemorrhagic manifestations. In the AEC region, dengue is one of the leading causes of severe illness among children and one of the most serious health concerns for expatriates. Now, with the availability of the first vaccine against dengue, 2016 could be a milestone year in the fight against this disease.
Dengue is caused by a virus transmitted by mosquitoes. Quite aggressive, these mosquitoes can bite throughout a 24-hour period but do so more frequently during daylight hours and can be recognised by the white markings on their legs.
There are four different strains (or serotypes) of dengue virus (from 1 to 4). Infestation with one strain confers lifelong immunity but only to that strain. It is thus possible to contract up to four dengue infections during one’s life.
The incubation period is usually from four to seven days. Most dengue infections cause no or few minor symptoms such as transitory fever for a few days. Typical dengue symptoms include severe headache, high fever, extreme weakness and back pain. There is often a measles-like, non-itchy rash over the face, thorax and limbs. The symptoms subside within 10 days but marked fatigue persists for a few more weeks.
About five to 10 per cent of infected patients develop the severe form of dengue, called dengue haemorrhagic fever, which is responsible for multiple bleeding. The first indication of bleeding is a haemorrhagic rash with small red spots found on any part of the skin. Bruising, bleeding from the nose or gums and, more worryingly, intestinal bleeding may occur. About 10 per cent of patients suffering from patent haemorrhagic complications progress to life-threatening dengue shock syndrome – popular Thai actor Tridsadee “Por” Sahawong is a recent case in point – and should immediately be admitted to a well-equipped intensive care unit.
Importantly, individuals who have suffered dengue a first time have an increased risk of a more severe dengue syndrome in the future compared with those who have not been previously exposed to the disease. This unusual and still poorly known phenomenon is referred to “antibody dependent enhancement” and represents an additional challenge in vaccine development.
Accurate diagnosis requires a simple test to detect the presence of dengue virus in whole blood through immunologic techniques.
There is no specific treatment for the dengue virus itself. Besides blood transfusion in high-risk patients, supportive therapy is provided to maintain the vital functions.
In a proper medical environment, the dengue mortality rate is below one per cent as long as the illness is recognised early. Dengue awareness among newly arrived expats is thus essential.
After 20 years of laudable research by the French pharmaceutical firm Sanofi, a vaccine is now available. It was registered in three countries last month and should hopefully be available this year throughout the whole AEC region.
Administered through three injections at six-month intervals, the vaccine efficacy rates vary from 50 per cent to 78 per cent depending on the causal viral strains.
When pooling the results of trials, the vaccine prevented the dengue infection in 60.3 per cent for all ages but was more pronounced among children aged nine years and older (65 per cent) than younger children (45 per cent). More importantly, vaccine efficacy against hospitalisation for dengue (admission due to more severe manifestations) was 80.3 per cent. There were no safety concerns.
However, in younger children (the under-nines), from the third to fourth year of follow-up after vaccination, it appeared that the risk of dengue infection became higher among those who were vaccinated compared with those who did not get the vaccine shots. As a result, the dengue vaccine is unfortunately indicated for adults and children above nine years of age only.
Even though the number of individuals over the age of nine is much higher than those younger than nine, these findings, if confirmed, might represent a significant drawback, not least because the severe forms of dengue are found predominantly in infants four to nine months of age and in children aged five to nine years.
That said, taking account of the potential complications of dengue infection and while waiting for the completion of follow-up studies, the available data show that vaccination is justified for individuals above nine living in endemic regions.
As dengue is indeed a public health concern, its effective control will now depend on the successful implementation of government-sponsored immunisation programmes throughout the country. |And this in turn will mainly depend on the political commitment of country’ health authorities to effectively control this endemic disease.

DR GERARD LALANDE is managing director of |CEO-Health, which provides medical referrals for expatriates and customised executive medical |check-ups in Thailand. He can be contacted at [email protected].