Air travel has come roaring back. Not everyone was ready. Customers are facing hourslong phone waits for assistance. Long lines have emerged as airlines, airports and the Transportation Security Administration scramble to hire staff and accommodate the influx of passengers.

Airline and airport executives say they anticipated that vaccines and easing restrictions would stoke renewed appetite for travel, but the speed and magnitude of the resurgence has exceeded their expectations. Even without the return of most business and international travel, the number of people passing through U.S. airports has surpassed two million on some days, a threshold last reached in March 2020. July 4—typically the peak of the summer travel season—is looming.

U.S. carriers are scheduled to fly more than 88 million seats in July, a 32% increase from April. That is still well short of 2019, but airlines are adding capacity much more quickly than they have in the past. Over the same four-month period in 2019, airlines increased the number of seats in the market by just 9% to meet summer demand, according to Cirium, an aviation-data provider.

Demand has quickly absorbed the extra seats. Planes are 83% full on average, and even more packed during heavily busy periods. Last year airlines offered steep discounts and deals. Now airfare is on the rise. The Labor Department reported last week that its airfare index rose 7% in May after gaining 10.2% in April. Carriers say leisure fares are on track to meet or exceed 2019 levels this summer.

The rapid increase has caused some growing pains.

U.S. airlines got $54 billion in government aid so they could keep paying their workers and avoid furloughs and layoffs that would make it more difficult for them to respond to rising demand when the time came. But carriers also encouraged many workers to retire early or take extended leaves of absence to stretch the aid as they faced a dire outlook last year.