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Previous infections with dengue virus may have protected some people in an urban slum in Brazil from getting Zika.

In a study of more than 1,400 people in the Pau da Lima area of Salvador, those with higher levels of antibodies against a particular dengue virus protein were at lower risk of contracting Zika, researchers report in the Feb. 8 Science. “The higher the antibody, the higher the protection,” says Albert Ko, an infectious disease physician and epidemiologist at the Yale School of Public Health.

That finding contrasts with previous studies in mice and in cells grown in lab dishes, in which antibodies against dengue seemed to make Zika worse