Would-be travelers who canceled flights last year may still be waiting for safer conditions before they return to the skies. They shouldn't assume their flight vouchers will be doing the same.

After travel shut down last spring, the number of people flying plunged in the United States, sometimes falling by millions in a single day. According to the U.S. Department of Transportation, those who no longer planned to fly were entitled to a refund only if the airline canceled their flight or changed it significantly.

That left masses of travelers with some kind of voucher or credit for future travel, with the future still very much unclear. In response to questions from The Washington Post, major U.S. airlines would not say how many outstanding travel vouchers or credits their customers were holding.

"Due to the sudden and severe disruptions to travel at the height of the pandemic and with travel demand slowly returning, we continue to have a large amount of travel credits outstanding," JetBlue spokesman Derek Dombrowski said in an email.

Despite the uncertainty over when travel will be safe again, those credits will expire - some of them soon. And it's up to consumers to stay on top of their options.

"I cannot stress enough the importance of reading the fine print on a voucher," said Scott Keyes, founder of Scott's Cheap Flights. "They are incredibly variable, not only airline to airline but even within the same airline."

United Airlines, for example, has electronic travel certificates and future flight credit. Future flight credits are issued to those who cancel a flight and want to rebook later. But there are several conditions: They are only good for the traveler whose name was on the reservation originally; the passenger has to pay the difference if the new ticket costs more than the original; and if the new ticket costs less than the original, the remaining value disappears.

American Airlines says it offers flight credit, travel vouchers and trip credit, all for different uses and with varying rules. The airline introduced trip credits in August for passengers who have extra value on a ticket they've voluntarily exchanged.

Spokeswoman Andrea Koos said the different types of credits expire at different times depending on when they were issued.

"However, we have extended voucher validity for customers during the pandemic and continue to work with customers to provide flexibility," she said in an email.

Keyes said travelers who are holding vouchers or credits should make sure they're aware of the expiration date - which could either be the date by which they need to travel or just make a new booking.

Jordan Staab, president of SmarterTravel Media, which includes the site Airfarewatchdog, said that if the expiration date is just the date by which to book a new trip, travelers can get creative.

"A really fun hack there is you can book a refundable fare on most airlines," he said, and change it again if the new date still doesn't feel safe. "You have essentially a credit for the future with that airline."

One caveat to keep in mind, however, is that passengers who are holding a voucher have no guarantee they'll find the same flight at the same price. Kaylin Portillo Chavez, 22, and her boyfriend were planning to go to Cabo San Lucas in June 2020 and have continually rescheduled the trip. Their airline, Volaris, requires passengers cover the difference in airfare.

"Right now I am scheduled to go to Cabo in late July but given that neither my partner nor I are vaccinated, we still want to wait until things are safer to go," she said in an email. "If they ask us to pay the difference in cost, we may have to eventually let the vouchers go."

Keyes said travelers should also make sure they're clear on whether they can use their vouchers for other travelers, or only themselves; whether they can expect to keep the remaining amount of the credit if their new flight is cheaper; whether multiple vouchers can be used for one new booking; if a credit can pay for things other than the fare, such as bags or seat selection; and whether the credit can be used on partner airlines. Staab also notes that some airlines are also allowing travelers to turn their credits into frequent flier miles.

Even though it's too late to change course from a year ago, Keyes said, he encourages anyone faced with the option of a cash refund or a credit to avoid the credit scenario. Vouchers or credits carry the risk of an airline going out of business or ending certain routes.

"If at all possible, always take the cash refund," he said. "It's so much simpler."