'They've jumped the gun': scientists worry about Russia's Covid-19 vaccine

Rising chorus of concern over Sputnik V vaccine stems from opaque development and lack of mass testing

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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.