The world's most advanced vaccine against dengue fever, being developed by French drugmaker Sanofi SA, proved only 30 percent effective in a large clinical trial in Thailand - far less than hoped.
The result leaves uncertain the future of a product that Sanofi has previously said could generate more than 1 billion euros ($1.3 billion) in yearly sales. But researchers said it did show for the first time that a safe vaccine was possible.
The disappointing outcome was down to the vaccine's failure to protect against one type of dengue virus, which turned out to be the prevalent one in Thailand at the time of the study.
Overall efficacy was 30.2 percent - statistically insignificant and well below the 70 percent-plus researchers had anticipated at the start of the company-funded trial.
The mosquito-borne disease, also known as "breakbone fever", is a threat to nearly 3 billion people and is caused by four types of virus, none of which confers immunity from the others.
"This result knocked me off my chair," Scott Halstead, a senior scientific adviser for the non-profit Dengue Vaccine Initiative, who was not involved in the study, told Reuters.
"This is a very sobering outcome and a lot of thought and further experimentation is going to have to be done in order to understand what happened.
"You can't be very confident that this vaccine is going to significantly reduce human illness."
Hopes had been high for the closely watched Thai trial, given the fact that dengue belongs to the same virus class as yellow fever and Japanese encephalitis, both of which are controlled with highly effective existing vaccines.
But it seems that making a mixed dengue vaccine containing four different virus strains can produce uneven results, underscoring the complexity of a disease that scientists have been trying to develop a vaccine against for over 70 years.