The higher a person's immunity to dengue virus, the lower their risk of Zika infection, according to a study which followed nearly 1,500 people living at the heart of the 2015 Zika outbreak in Brazil.
The study, published in the journal Science, also provides evidence that Brazil's Zika epidemic has largely petered out because enough people acquired immunity to reduce the efficiency of transmission.
"Take that with a grain of salt, though," said Ernesto TA Marques, an associate professor at the University of Pittsburgh in the US.