An American Airlines jet prepares to depart at Reagan National Airport. Customers who are pregnant or traveling with someone who is pregnant to a country listed on the Centers for Disease Control and Prevention travel advisory you can request a refund. (John McDonnell / The Washington Post)

Four major U.S. airlines have announced that they will allow some customers to change or cancel flights to countries where the Zika virus is active, a move that reflects the growing worries about the impact of the virus.

American Airlines said pregnant women and their companions would be allowed to request a full refund if they have a doctor's note.

United's and Delta's policies are broader. They allow any concerned customer scheduled to go to a country listed in a Centers for Disease Control and Prevention travel advisory to rebook to a later date or get a full refund.

And Southwest, which recently expanded its routes to Central America and the Caribbean, said that its normal policy is to offer changes without a fee and that it would continue to allow customers impacted by Zika to do so.

The countries and territories named in the CDC travel advisory include Barbados, Bolivia, Brazil, Cape Verde, Colombia, Dominican Republic, Ecuador, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Puerto Rico, Saint Martin, Suriname, U.S. Virgin Islands and Venezuela.

Many smaller carriers, as well as cruise lines and tour providers, such as Carnival, are also offering no-fee rebooking or refunds.

While it's too early to assess the financial impact of Zika on the travel industry, Helane Becker, an analyst with Cowen and Co. who follows the airline industry, wrote in a research note this week that she believes there could be "a slowdown in bookings during this time of heightened media coverage and general fear until more information is known."

Here's a roundup of what different companies have announced:

American Airlines

The airline initially said the refund policy would only apply to five countries, but it later expanded it to include Puerto Rico, Martinique and four others.

"If you are pregnant and scheduled to travel to a destination outside the U.S. that is affected by Zika virus, you and your travel companions can request a refund. To qualify, you must provide a doctor’s note confirming your pregnancy and stating your inability to travel due to Zika virus."

United Airlines

Applies to all countries on CDC list.

"Customers planning travel to a country that has been impacted by the Zika virus are advised to contact the United Customer Contact Center with questions or to change an itinerary. For the latest information and updates on travel recommendations, please visit the CDC Zika virus travel information page."

Delta

Applies to all countries on CDC list.

"Customers with current reservations who are concerned about travelling to destinations reported by the CDC to be affected by Zika Viral Illness should call 1-800-221-1212 (U.S.) or your local Reservations office and speak with a Delta Representative.

Customers may qualify for a change to alternate destinations, travel dates or a refund. Customers may make fee-waived changes to future reservations/tickets. However, changes need to be made by February 29, 2016."

LATAM

Applies to all countries on CDC list.

"For pregnant passengers that have already initiated their trips to the aforementioned destinations, they can return early, subject to seat availability, at no extra charge."

Norwegian Cruise Line Holdings and Carnival Corp.

Pregnant women will be allowed to reschedule cruises or change to another itinerary in a region that is not affected by Zika.

This post has been updated.

Here's a look at the pandemics that made it to our shores. (Gillian Brockell/The Washington Post)

Read more:

WHO: Zika virus ‘spreading explosively,’ level of alarm ‘extremely high’

FAQ: What is Zika, and what are the risks as it spreads?

CDC issues interim Zika guidelines for testing infants

Why the United States is vulnerable to the alarming spread of Zika virus