The Aedes aegypti mosquito is spreading the Zika virus throughout the Americas. (Oscar Rivera/EPA)

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.

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.

Source: CDC

Source: CDC

Source: CDC

Source: CDC

Source: CDC

But Brazil, the epicenter of the epidemic and the host of the 2016 Summer Olympics, does not. The mosquito-borne virus has been linked to a rare condition known as microcephaly, in which children are born with abnormally small heads and underdeveloped brains. The condition has surfaced in hundreds of babies in Brazil.

Source: CDC

This is the first time the CDC has provided such a caveat in its travel alerts, Cetron said. Since the agency's initial advisory in January, when it cautioned pregnant women against traveling to Zika-affected regions, CDC has received comments from residents and governments in some affected countries noting that their outbreaks were not "intense," Cetron said.

"What we're trying to do is be scientifically responsible and accurate about where the risk is really minimal and provide the most important advice that people can use," he said. "We know that mosquitoes don't exist at the top of Mount Everest. The question is, how low can you go and what is the threshold where you are unlikely to find these mosquitoes?"

Earlier this week, the World Health Organization also urged pregnant women to avoid travel to Zika-affected areas, but said it remains up to individual countries to designate regions where there are ongoing outbreaks.

The CDC released 37 destination-specific Zika travel notices that include elevation maps for locations above and below 6,500 feet.

But mosquitoes are not the only way the virus is spread. Sexual transmission is more common than researchers initially thought. Of 193 cases confirmed in the continental United States, six include cases where travelers passed the virus to partners through sex.

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