With an estimated 390 million infections each year, dengue is the world’s fastest growing mosquito-transferred disease. Around half the global population – 3.9 billion people across 128 countries, mostly in Asia, Latin America and Africa – are at risk of infection.
Vaccine manufacturers have been working on a solution to this public health crisis for over 20 years, and Sanofi Pasteur recently completed promising Phase III trails of Dengvaxia in Asia and Latin America.
But while it’s shown to be effective in some situations, Dengvaxia isn’t a silver bullet for dengue.
Dr Stefan Flasche, an Assistant Professor at London School of Hygiene & Tropical Medicine, worked with an international team of dengue experts to use mathematical models to predict the long-term public health impacts of Dengvaxia. Their findings showed that Dengvaxia’s effectiveness in combatting severe dengue could all depend on where it’s used.
In areas where dengue is common – where at least 70 per cent of people contract it – the routine vaccination of 9-year-olds could be a smart move.
“If targeted only to those populations with a high burden of disease, the vaccine has the potential to prevent about 1 out of 5 dengue hospitalisations and be good value for money”, Flasche told SBS Science.
But where the disease is rare – where less than 30 per cent contract it – the vaccine could actually increase hospitalisations for dengue.
“The risk only occurs if the vaccine is administered to individuals who haven’t been infected before and are living in areas with little dengue burden”, he said.
How the vaccine works
To understand why Dengvaxia is predicted to lower dengue burden where the disease is common, but raise it where it’s rare, we need to look at how the disease works.
The breakthrough moment for Dengvaxia came when clinical trials in over 30,000 people showed that the vaccine “has the potential to protect against all four serotypes of dengue”, Flasche said.
Dengue is caused by four different ‘serotypes’ of the dengue virus, creatively named DEN-1, DEN-2, DEN-3 and DEN-4. If you contract then recover from one serotype, you have lifelong immunity against it.
Usually, a person’s first bout of dengue is mild, presenting flu-like symptoms or, often, no symptoms at all. But if a subsequent infection occurs with another dengue serotype (for example, you first had DEN-2, now you’ve got DEN-4), the risk of developing ‘severe dengue’ sharply increases.
Severe dengue (also known as dengue hemorrhagic fever) comes with serious health risks; swelling, organ impairment, breathing difficulties, and bleeding cause intense suffering and can ultimately prove fatal. The WHO estimates that 500,000 people are hospitalised with severe dengue each year, with around 2.5 per cent dying from their condition.
For patients who’ve never had dengue, Dengvaxia can act to their immune system as their first infection. This means that their first natural dengue infection – one that they contract through a mosquito bite – will act like their second, putting them at a greater risk of severe dengue.
In light of this finding, “the World Health Organisation has been leading discussions with policymakers on the specifics how to make best use of Dengvaxia”, Flasche said.
Dengue viruses are primarily spread by Aedes aegypti, the ‘yellow fever mosquito’, which also transmits Zika and chikungunya, and these diseases are largely restricted to its tropical and subtropical distribution. But a recent study suggests their habitat could expand under climate change, with Australia, Europe and North America expected to see the largest increases in human exposure to the mosquito.