RACHEL MARTIN, HOST: There is some good news about Zika. Health organizations are reporting fewer cases of Zika-linked birth defects than projected. But, as NPR's Michaeleen Doucleff reports, the findings are causing researchers to take a closer look.</s>MICHAELEEN DOUCLEFF, BYLINE: In 2015, Brazil had a massive outbreak of Zika, and with it came a surge in birth defects. Thousands of babies were born with very small heads, or microcephaly. So when Brazil had another outbreak of Zika in 2016, scientists expected another surge in microcephaly. But Chris Dye at the World Health Organization says that never happened.</s>CHRIS DYE: We predicted over a thousand cases of microcephaly when, in fact, we have less than a hundred.</s>MICHAELEEN DOUCLEFF, BYLINE: I mean, the difference between 2015 and 2016 is actually quite dramatic, isn't it?</s>CHRIS DYE: It's spectacular, so this is a huge discrepancy.</s>MICHAELEEN DOUCLEFF, BYLINE: Dye and his colleagues say in the New England Journal of Medicine that it's unclear why. But one possibility is that researchers overestimated the number of Zika cases in Brazil last year - that the virus wasn't as big of a problem as they thought, so microcephaly wasn't as bad. That's what Dye thinks.</s>MICHAELEEN DOUCLEFF, BYLINE: But Albert Ko at Yale University says there's another possibility. Zika may not be working alone. Instead, another infection may make Zika worse and increase the risk of microcephaly. In particular, Ko is concerned about a virus called dengue, which is common in Brazil.</s>ALBERT KO: This prior exposure to dengue may actually enhance or promote the risk of birth defects.</s>MICHAELEEN DOUCLEFF, BYLINE: Ko is investigating this possibility right now. And if it turns out to be true, then the global threat posed by Zika might be less dire.</s>MICHAELEEN DOUCLEFF, BYLINE: Michaeleen Doucleff, NPR News.
Scientists expected a surge of severe birth defects in Brazil because of the Zika outbreak. But that didn't happen last year. Researchers are re-examining the link between Zika and birth defects.