U.S. health officials have come up with an exception to their travel warnings for the Zika virus: altitude.

The Centers for Disease Control and Prevention (CDC) says that the mosquito that spreads the Zika virus is rare above 6,500 feet (2,000 meters). Therefore, it is OK for pregnant women to travel to destinations at high elevations like Mexico City.

But the updates will not make a big difference in most affected countries.

The CDC has advised pregnant women to avoid travel to a total of 37 countries and territories because the Zika virus is the suspected cause of a surge of birth defects in Brazil.

Zika virus could also be related to an increase in cases of Guillain-Barré syndrome, an immune disorder that can cause paralysis; however, travel advisories have not been based on the possible threat of the syndrome.

The countries currently included in the CDC’s travel notices include:

• In the Caribbean: Aruba; Barbados; Bonaire; Curaçao; Dominican Republic; Guadeloupe; Haiti; Jamaica; Martinique; the Commonwealth of Puerto Rico, a U.S. territory; Saint Martin/Sint Maarten; Saint Vincent and the Grenadines; Trinidad and Tobago; U.S. Virgin Islands

• In Central America: Costa Rica; El Salvador; Guatemala; Honduras; Nicaragua; Panama

• In the Pacific Islands: American Samoa; Marshall Islands; New Caledonia; Samoa; Tonga

• In South America: Bolivia; Brazil; Colombia; Ecuador; French Guiana; Guyana; Paraguay; Suriname; Venezuela

• Cape Verde

• Mexico

The CDC on Friday posted maps showing which areas in each country are above 6,500 feet. In Mexico, many areas are above 6,500 feet and are therefore considered safe, including Mexico City.

Guatemala also has significant area above the elevation, but major international airports are below.

In some countries, the elevation suggestion is of little use. Brazil, which is banking on the summer Olympics to bring tourists to the country, has only a few peaks above 6,500 feet.

And Nicaragua also has only a few peaks above the “safe” elevation. 

Additional reporting by The Associated Press.

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