[This is a slightly different version of a story we wrote for NBC News]

When the pandemic hit, Vanessa Mumford-Minshull of Campbell, CA scrambled to cancel a multi-leg, multi-airline European trip for four people. She asked for refunds. Airlines automatically issued vouchers. “I sent email saying, ‘I don’t want this,’ but got no answer,” says Mumford-Minshull, who spent days calling, writing, and documenting her efforts before finally getting refund help thanks to her credit card company. “With the vouchers, there were too many unknowns due to COVID-19,” she said, “I could have lost almost $4,000.”

Millions of other travelers who accepted or were issued travel credits or vouchers are pulling the emails out now that a year has gone by, vaccination rates are on the rise, and cities are reopening to visitors. Many hopeful flyers are discovering their credits will expire before they get to use them. Others will find that the vouchers have restrictions that make them difficult to use

Besides dashed travel hopes, there is a great deal of money at stake. Companies still deliberating when to restart non-essential business travel may be sitting on hundreds of thousands of dollars of unused travel credits, notes corporate travel management company TripActions. And airlines, which recently received billions of dollars in a third round of federal pandemic funding relief,  are holding billions in unused vouchers.

“I can say with confidence there are more outstanding travel vouchers right now than have existed ever before, given the number of trips that got disrupted simultaneously because of the pandemic,” says Scott Keyes, founder of Scott’s Cheap Flights.

What to do to if you have pandemic travel credit or flight vouchers

If you accepted or received airline travel credits or vouchers for a trip canceled due the pandemic, experts say check the expiration dates right away and start reading the fine print. If the expiration date has not passed and you do not have travel plans just yet, call and ask the airline to extend the date. (It is worth a try even if the expiration date has just passed). Given the Centers for Disease Control’s advice to continue avoiding non-essential travel and shifting COVID-19 hotspots, airlines may be willing to do this.

“For those consumers who have unused vouchers, it’s critical that they stay on top of this and be aware of the expiration dates,” says William McGee, Aviation Adviser to Consumer Reports, which logged more 700 consumer complaints and stories about vouchers and refunds in less than a week.

To try to help travelers figure out their options, McGee says he did a deep dive into policies being offered by ten U.S. carriers and found that the best and most transparent policy was from Allegiant Air, which set the expiration for its travel vouchers at two years from the initial date of purchase. He says other policies “are confusing, and the expirations can vary greatly based on date of booking, date of travel, date of cancellation,” and other factors.

Here are links to the current rules and policies of Alaska Airlines, American Airlines, Delta Air Lines, Frontier Airlines, Hawaiian Airlines, JetBlue, Southwest Airlines, Spirit Airlines, and United Airlines. You will notice for example, that American Airlines has 3 types of travel credit – Flight Credit, Travel Vouchers, and Trip Credit – and that United Airlines’ travel certificates are now good for up to two years after the date they were issued.

Some other tips to keep in mind

While you should be able to use your travel credits or vouchers to book the same or a different itinerary, you will be shopping for your new tickets at new prices. And, as travel picks up and pent-up demand for travel increases, you may need to throw extra money in the pot to get the flights you want.

If you know where you want to go but are not sure when you will feel comfortable flying, the experts at Scott’s Cheap Travel suggests you use your travel credits to book a flight on an airline that is still offering free changes on all fares. “While you’ll have to pay the difference if the fare rises, “you’ll be able to change dates without an extra fee and you won’t lose the value of your voucher.”

Beyond that, “our biggest advice,” says McGee of Consumer Reports, “is to stay persistent.” This was Consumer Reports’ advice last year when advising consumers about airline refunds at the start of the pandemic and the same holds true with vouchers a year in, says McGee. “It may take four or five or even more emails, calls, and letters,” but the success stories come from those who are persistent.