By Dr Eddy Bajrovic, Medical Director, Travelvax Australia
What’s the number one health risk for the average Australian traveller visiting a tropical destination?
Ten years ago the answer would have been traveller’s diarrhoea: Today, it’s almost certainly dengue fever.
To date this year, 1057 Aussies have returned home from an overseas trip infected with the mosquito-borne virus – most having been bitten by an infected female Aedes aegypti or Aedes albopictus mosquito while visiting Asia. A small number were infected in areas of North Queensland, where localised outbreaks occur each year.
Over the past two decades, the tally of Australian cases recorded annually has been steadily rising. We appear likely this year to pass last year’s total of 1840 – itself a 23-year high.
Fortunately, despite the local cases, Australia has escaped the worst of the dengue scourge.
Globally, 390 million people are infected each year and almost 25% (96 million) of them suffer its very painful, debilitating symptoms, which can take weeks – even months – to recover from.
The rise and rise of dengue
The WHO estimates that the global burden of dengue has risen 30-fold over the past 50 years, with more than half of the world's population now at risk. The WHO aims to reduce dengue illness by half and deaths by at least 25% by 2020.
As yet, there is no vaccine available to treat or prevent dengue fever: All that travellers can do to minimise their risk of infection is to take steps to avoid mosquito bites throughout the daylight hours, but especially around dawn and dusk.
An effective personal insect repellent applied regularly as required during the day is the most important ‘weapon’ in their arsenal.
Efforts to develop a vaccine have been complicated by the fact that the disease is caused by 4 distinct virus ‘serotypes’ – DENV 1, 2, 3, and 4. To be truly effective, the vaccine must target all four.
First vaccine on the horizon
French pharmaceutical company, Sanofi Pasteur, has spent 20 years and $US1.5 billion developing its dengue vaccine candidate (CYD-TDV). The vaccine is the first to reach phase 3 clinical testing in humans.
However, the new vaccine has achieved only moderate protection (56%) against the disease in Asian children, according to new research published in The Lancet.
More encouraging, it was nearly 90% effective against the most serious, haemorrhagic form of the disease, while reducing dengue-related hospitalisation by 67%.
The trial involved 10,275 children aged two to 14 years in Indonesia, Malaysia, the Philippines, Thailand, and Vietnam.
Of these, 6851 received three injections of CYD-TDV six months apart, while a control group of 3424 kids did not receive the vaccine. The results were measured over two years.
Significant impact on global burden
A total of 250 dengue cases were identified more than 28 days after the final injection, 117 (1.7%) in the vaccine recipients and 133 (3.9%) in the controls, for an overall efficacy rate of 56.5%.
Some commentators have described the results as disappointing. Certainly, everyone would prefer to see higher levels of protection.
However, reducing the incidence of symptomatic dengue infection by more than half and reducing severe disease and hospitalisations even further is a ‘glass half full’ result.
Clearly, CYD-TDV could have a significant impact on public health around the developing world, given the high burden of disease in countries where dengue occurs.
Also, it was particularly noteworthy that the vaccine’s protection was almost as high after one or two doses as after three.
If the vaccine can be just as effective with fewer doses, immunisation programs would be more affordable, as well as being more convenient for recipients.
Personal protection vital
The Sanofi vaccine is the most advanced of a number being developed in the hope of controlling dengue fever. It remains to be seen if any of the others achieve higher levels of protection.
It’s more likely that dengue control will rely on vaccines in combination with other strategies, including new medications, better management of cases, improved insecticides, and more effective approaches to controlling mosquitoes breeding.
Vaccine or no vaccine, personal protection measures against mosquito bites should never be ignored and will remain the cornerstone of prevention.
And, with Chikungunya fever also posing a greater risk to humans living in and visiting the tropics, it is hard to imagine any time when we won’t need to take personal responsibility for preventing infection.