LONDON—European air travel is finally returning, thanks to a barrage of cheap tickets from discount carriers and the relatively smooth rollout of a continentwide vaccination-certification system.

Until just recently, U.S. and China domestic air-travel markets have been booming, with passengers taking advantage of what had been a fall in Covid-19 cases in both places. In the U.S., a robust vaccination drive also boosted traveler confidence.

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Intra-continental travel in Europe, though, has remained stuck far below pre-2019 levels, largely because the market is so heavily dependent on cross-border flights. Those have been curtailed by government restrictions meant to contain the virus, including quarantine requirements, testing and vaccinations.

Now, the fast-spreading Covid-19 Delta variant has started to dent U.S. and China travel, and Europe has become an aviation industry bright spot—at least for now.

Last week, intra-European flights were down 27% compared with the same week in 2019, according to an analysis by aviation consulting firm Cirium. That is the smallest gap recorded since March 2020, when airlines started curtailing service amid the first global wave of Covid-19 infections.

Meanwhile, U.S. capacity was down 15% compared with 2019 levels, after coming close to matching pre-pandemic levels at the beginning of July. Capacity then was down just 7.6%, before the Delta variant started spreading widely.

Startups, governments and nonprofits are racing to create so-called “vaccine passports,” or digital health passes aimed at helping people travel and safely move around in public. WSJ explains what it would take to get a global digital health pass system off the ground. Illustration: Zoë Soriano The Wall Street Journal Interactive Edition

Southwest Airlines Co. said earlier this month that the variant was slowing bookings and had triggered an increase in cancellations. Both Frontier Airlines and Mesa Air Group Inc. —one of the biggest regional carriers—also have flagged concerns about the variant’s impact on demand.

In China, flights were down 45% from 2019 levels last week. In early July, China capacity was 10% higher than 2019 levels, according to Cirium.

The recovery in Europe comes as discount carriers, like Ryanair Holdings PLC and Wizz Air Holdings PLC, increase capacity and open new bases and routes. Discounters are hiring pilots and crews to take advantage of pent-up travel demand over the critical summer period.

“We are selling a lot of cheap seats to recover the market very quickly,” Ryanair Chief Executive Michael O’Leary said this week. He said bookings for the remainder of the year were “very strong.”

Aviation executives and analysts say the rollout of the European Union’s digital health pass and quickly growing levels of vaccinations across the continent have also been critical to Europe’s short-haul recovery. The EU has created a bloc-wide digital-certification system that allows passengers to prove they are either vaccinated or have recently tested negative for Covid-19.

The system had drawn skepticism over how it might be accepted and how it would work in different countries. So far, however, it has helped reopen borders even as waiting times at airports have lengthened because of the additional checks, executives and analysts say.

”That has become a booster for European travel,” said Wizz Air Chief Executive József Váradi, “That’s the fundamental factor—that if you are fully vaccinated or you are tested negative you actually can travel.”

Travelers’ vaccination status was checked Aug. 12 at Eindhoven Airport, in the Netherlands.

Photo: Hollandse-Hoogte/Zuma Press

Wizz Air’s capacity is at 103% of where it was this time in 2019, he said. The airline has hired 400 additional crew in August and is recruiting another 1,000 over the next four months.

He said revenue was down about 20%, though, because of lower ticket prices and because the airline was flying aircraft with more empty seats. Load factor, a measure of how full each flight is, was down about 10 percentage points, at about 85%, from 2019.

Like some U.S. carriers, Wizz Air has battled with delays at airports and with ground handlers as it tries to quickly ramp up after 18 months of depressed traffic. As a result, the airline has been forced to scrap some flights.

Write to Benjamin Katz at ben.katz@wsj.com