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 t