In 1977, Scott Halstead, a virologist at the University of Hawaii, was studying dengue fever when he noticed a now well-known but then unexpected feature of the disease.
Animals that had already been exposed to one of the four closely related viruses that cause dengue and produced antibodies to it, far from being protected against other versions, became sicker when infected a second time, and it was the antibodies produced by the first infection that were responsible, allowing the second infection to hitchhike into the body.
““[The] benchmark [of phase-3 testing] is crucial because a weak vaccine might be worse than no vaccine at all,” she said. “We do not want people who are only slightly protected to behave as if they are invulnerable, which could exacerbate transmission.”