United Airlines will allow customers who hold tickets to regions impacted by the Zika virus to postpone their trips or receive full refunds, a spokesman said on Tuesday.

United, the second-largest U.S. airline by capacity, said the offer corresponds to virus-hit areas specified by the U.S. Centers for Disease Control and Prevention. On Monday, the World Health Organization warned that mosquito-borne Zika is likely to spread to most of the Americas, sparking concerns by travelers on social media.

(Reporting By Jeffrey Dastin in New York)