A new vaccine against dengue falls short of its goal, the vaccine failed to protect against all its viral strains, report researchers.
Tested among just over 4,000 children in rural Thailand who were badly exposed to the mosquito-borne fever, the vaccine had no side effects but only worked against three out of the four dengue strains.
The outcome shows that the quest is far from over, but the good news is "a safe vaccine against dengue is possible", the researchers said in The Lancet.
According to the UN's World Health Organisation (WHO), between 50 and 100 million dengue infections occur each year in more than 100 countries. In 1970, the disease was endemic in just nine countries.
Half a million people each year develop severe dengue, a feverish condition requiring hospitalisation.
But the search for the vaccine has been complicated by the fact that there are four different strains -- known as DENV 1, 2, 3 and 4 -- all of which may circulate in an outbreak zone.
Adding to the problem is that dengue seems to be exclusive to humans, which means it is impossible to test vaccines on lab animals first.
As a result, trials have to proceed with even greater caution than usual, and the formula has to be built up incrementally, on what has previously been tested.
The candidate vaccine, CYD-TPV, was given to 2,669 children in three doses, while 1,333 were given a harmless lookalike called a placebo to act as a comparison.
The trial was a so-called Phase IIb, meaning a stepped-up test in the long three-phase process by which new drugs are tested for safety and effectiveness.
In the vaccine group, 76 children, or 2.8 percent, fell diagnosed with dengue, compared with 58 cases, or 4.4 percent, among the control group.
But among the inoculated children, protection against DENV 1, 3, 4 was 55.6 percent, 75.3 percent and 100 percent -- but against DENV 2, it was just 9.2 percent. Overall, the protection was just 30.2 percent.
French pharmaceutical giant Sanofi Pasteur said that despite this disappointment, the story was not yet over.
Phase III trials of CYD-TPV are underway in several other countries, where different conditions may give a different outcome or, possibly, tips for a solution to widen the vaccine's shield.