Traveling this summer has become a nightmare experience for millions of travelers in the U.S. and Europe.

Delays and cancellations are rampant, as are long lines and lost luggage. According to David Slotnick, senior aviation reporter at The Points Guy, there are a number of factors at play, but COVID is at the root of it.

“It all stems from the height of the pandemic, when companies — both airlines, airports, and their subcontractors — laid off or furloughed a lot of employees,” Slotnick said on Yahoo Finance Live (video above). “It’s been very difficult to staff up. We’ve obviously seen that here, too. But it’s been a difficult situation in Europe.”

Between April and June, European airline carriers had more than double the number of cancellations of U.S.-based ones, according to RadarBox.com. Lines at some airports in Europe have gotten so long that it has taken hours for some travelers to get through security and finally board their flights.

On Tuesday, John Holland-Kaye, CEO of London’s Heathrow Airport, went so far as to ask airlines to stop selling tickets until September.

“We recognize that this will mean some summer journeys will either be moved to another day, another airport, or canceled,” Holland-Kaye wrote in a letter to passengers. “And we apologize to those whose travel plans are affected.”

While no airlines have yet responded to the CEO’s request, many of these companies are aware of the problem at hand. Delta CEO Ed Bastian recently apologized to customers for the number of disruptions, though the airline has improved its reliability so far in July.

'It's been hard for them to hire'

Staffing shortages have been plaguing airlines as consumer demand has rebounded after a devastating drop early on in the pandemic.

Pilots and air traffic controllers are in short supply, as it can take years to be certified to work with large commercial airliners. In the U.S., many Delta pilots picketed at airports while the company renegotiates contracts with their labor union. They aren’t the only workers experiencing staffing issues, however.