BARRANQUILLA, Colombia — This tropical city on the Caribbean coast may hold the answer to one of the deeper mysteries of the Zika epidemic: Why has the world’s second-largest outbreak, after Brazil’s, produced so few birth defects?

In Brazil, more than 2,000 babies have been born with microcephaly, abnormally small heads and brain damage caused by the Zika virus. In Colombia, officials had predicted there might be as many as 700 such babies by the end of this year. There have been merely 47.

The gap has been seen all over the Americas. According to the World Health Organization, the United States has 28 cases — almost all linked to women infected elsewhere. Guatemala has 15, and Martinique has 12.

Had the rest of the Americas been as affected as northeastern Brazil, a tidal wave of microcephaly would be washing over the region. Most experts say that will not happen, but they are at a loss as to why.