This post originally appeared in Scientific American.

ATLANTA—Zika infection during pregnancy can lead to birth defects except, of course, when it does not. Now scientists are wondering why the virus catastrophically affects some fetuses but not others.

In Colombia, where the number of known Zika infections is second only to Brazil, there have been relatively few cases of related birth defects: 57 compared with more than 2,000 in Brazil, according to the World Health Organization. The U.S. has the third-highest number of Zika-related birth defects, with 31 combined cases and lost pregnancies due to miscarriage. The exact ratio of Zika infections to birth defects in each country remains difficult to determine, partly because so many Zika patients do not appear symptomatic, but what is clear is that birth defect rates are uneven.

That inexplicable geographic variability fueled speculation among scientists attending the American Society of Tropical Medicine and Hygiene Annual Meeting in Atlanta this week about what is causing the disparate patterns of Zika-related birth defects. Some answers may come from comparing the northeastern part of Brazil, where birth defects are the most common, to the rest of Brazil and Colombia, says Pedro Fernando da Costa Vasconcelos, director of Brazil’s National Reference Laboratory for Arboviruses.

One notable variance is that in northeastern Brazil few people receive vaccinations against the yellow fever virus whereas that is the norm in the other two locations, da Costa Vasconcelos says. Researchers need to study that disparity, he adds, because the yellow fever and Zika viruses are closely related, so yellow fever vaccination might provide some cross-reactive protection. Such an analysis should be starting soon, he says.

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