The Centers for Disease Control and Prevention now says pregnant women can safely travel to Zika-affected countries if they stay at elevations higher than 6,562 feet, where they’ll find few mosquitoes that could spread the virus.

But the agency’s revised travel advisory, released Friday, continues to recommend that trips to lower-elevation areas be avoided because the greater presence of mosquitos increases the risk of infection.

Until Friday, the CDC was cautioning pregnant women from traveling to the 37 countries and territories, primarily in the Americas, where the virus has spread rapidly.

We will update you when news breaks about the virus.

Because distribution of mosquitoes varies within a country's borders, depending on factors such as temperature, rainfall and population density, officials said they wanted to develop more precise guidance.

It comes just before spring-break travels for many families who might be contemplating destinations at higher altitudes, said Martin Cetron, the senior medical officer who authored the report.

About 40 million people travel from the United States to the Zika-affected regions each year, including about half a million pregnant women, CDC officials.

For its latest advisory, the agency looked at historical reports of where the Aedes aegypti mosquito is most likely to live. Researchers used elevation as a proxy for temperature because temperature varies widely and is difficult to predict locally. They used cases of dengue, a related viral disease also carried by the same mosquito, as a proxy for Zika.

In 16 countries where Zika virus is spreading locally, the risk of getting infected is likely to be minimal at elevations beyond 6,562 feet (2,000 meters) because mosquitoes are "predicted to be largely absent," the CDC determined. Countries with areas of higher elevation are: Bolivia, Brazil, Colombia, Costa Rica, Dominican Republic, Ecuador, El Salvador, Guatemala, Guyana, Haiti, Honduras, Jamaica, Mexico, Nicaragua, Panama and Venezuela.

Five countries--Bolivia, Colombia, Ecuador, Guatemala and Mexico--have substantial areas with higher elevation. The biggest, in terms of destination and location and population, is Mexico.