A prototype vaccine for dengue yielded lukewarm results two years ago, after wider trials it has proved more effective and is a potential arm against the disease researchers said Friday.
Devised by the French pharmaceutical giant Sanofi Pasteur, the so-called CYD-TDV vaccine provided only 30 percent protection against the dangerous fever when first tested among children in Thailand.
Widened to trials in four other Asian countries, where disease conditions vary greatly, the vaccine's protection has been shown to be significantly higher, at 56.5 percent overall, the scientists said.
The result falls short of the benchmark set by classic vaccines such as those for polio and measles, which can be more than 99 percent effective.
One reason for this is that CYD-TDV performed poorly against one of the four strains of dengue virus, the investigators reported in The Lancet.
These strains, or serotypes, circulate simultaneously, which means a vaccine should ideally protect against all of them.
Even so, the prototype was safe and well tolerated and its shield, if only partial, means it should be enlisted in the fight against dengue, they argued.
"Our results suggest that vaccination with CYD-TDV can reduce the incidence of symptomatic dengue infection by more than half and importantly reduced severe disease and hospitalizations," said Maria Rosario Capeding from the Philippines' Research Institute for Tropical Medicine.
"This candidate vaccine has the potential to have a significant impact on public health in view of the high disease burden in endemic countries."
Dengue is a potentially fatal fever, caused by a virus transmitted by a mosquito when it takes its blood meal, and is especially dangerous for children.
The virus infects around 390 million people each year, of whom about 96 million fall sick, according to UN estimates.
It was once considered a disease of the tropics that was endemic in only nine countries.
But globalization, climate change and jet travel are helping it to move into more temperate zones.
The World Health Organization (WHO) suggests that cases of dengue have risen 30 fold over the last 50 years, and more than half of the world's population are at risk of the disease.
- Wider trials -
The CYD-TDV vaccine was tested as a so-called Phase IIb trial among just over 4,000 children in rural Thailand, the results of which were reported in September 2012.
The new figures are those of a Phase III trial -- normally the final step in the process to test new drugs for safety and efficiency -- carried out in Indonesia, Malaysia, the Philippines and Vietnam, as well as in Thailand.
More than 10,000 children aged two to 14 years were enrolled. They were randomly assigned to receive three injections of the vaccine or a placebo over 12 months, and were followed for up to two years.
During this period a total of 150 dengue cases occurred, a majority of them in the placebo group, demonstrating an overall effectiveness of 56.5 percent.
But the protection varied according to the serotype -- more than 75 percent against virus types 3 and 4; 50 percent against type 1; but only 35 percent against type 2.
On the plus side, those who had received the vaccine were also far less likely to fall ill with a severe form of the disease, dengue haemorrhagic fever, which leads to half a million hospitalizations each year.
In a commentary, Annelies Wilder-Smith, a professor at Singapore's Nanyang Technological University, said a vaccine that halved annual cases of dengue "would present a significant public health benefit" but was not a magic bullet.
"For the moment, the CYD-TDV vaccine is the best we have; however, with 56 percent efficacy it will never be a single solution," Wilder-Smith said.
Other strategies, including better approaches to tackling mosquitoes that cause the problem, would also have to be part of the campaign, she said.
The children in the trial are being followed up for another four years to see whether the vaccine's promise still holds up.