A Dengue vaccine program was recently suspended in the Philippines due to concerns that the “vaccine could worsen the potentially deadly disease in people not previously infected.” The vaccine, known as Dengvaxia, has been given to over 830,000 children. It’s the first-ever approved dengue vaccine, produced by the French pharmaceutical company Sanofi. While it appears to be effective in people who have already had the virus, given the results of the study, the vaccine program has been suspended pending a review.

Dengue is an infection caused by mosquitoes. It’s a flu like illness found in tropical and sub-tropical climates and can be deadly. It’s the leading cause of serious illness and death among children in these areas. Infection rates have grown globally in recent decades and half the worlds’ population is at risk. Early detection and access to proper medical care dramatically lowers fatality rate.

Did a researcher foresee this problem?

Four decades ago, Dr. Scott Halstead, a leading figure in dengue research, first proposed that antibodies from an initial exposure to one of four types of the disease could increase the risk of a potentially lethal complication called severe dengue when a person was infected a second time, a process know as antibody-dependent enhancement or ADE.

This phenomenon could make development of a dengue vaccine tricky.

Rather than being protective, a shot given to someone who had never had dengue could act like a first infection, increasing their risk of severe dengue when they were exposed a second time.