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

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 already produced by the first infection that were responsible, allowing the second infection to hitchhike into the body.

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