Major U.S. airlines are increasingly mandating Covid-19 vaccines for employees. Some of the regional carriers that fly under their names have not — at least not yet.

Large airlines like United, American and Delta contract smaller carriers to operate some shorter routes in their networks, tickets for which are sold on the bigger carrier’s website and other platforms as its own. Some of these smaller airlines are wholly owned subsidiaries such as American’s Envoy and Delta’s Endeavor Air while others, like SkyWest and Mesa Air Group, are independent.

Representatives for Mesa, Envoy, SkyWest and United contractor CommutAir said they haven’t issued requirements for employees yet but are evaluating whether federal requirements will compel them to mandate vaccines for staff.

The Biden administration last month said federal contractors must ensure that their staff is vaccinated against Covid by Dec. 8. President Joe Biden also said that companies with more than 100 employees will have to enforce a vaccine mandate for all staff, or regularly test those who aren’t. Regional carriers would at the very least fit into the second category.

“It’s somewhat murky,” Jonathan Ornstein, CEO of Mesa Air Group, which flies for American and United, said of what will be required of regional airlines. The Phoenix-based carrier’s executives are awaiting more clarity on the rules from federal officials, lawyers and airline customers, but the carrier will comply with federal mandates, he said.

Ornstein said he worries about potentially losing workers if the company of more than 3,000 employees mandates staff vaccinations without a federal requirement that would put competitors on a level playing field.

“I’m concerned that if in the hypothetical we were to mandate a vaccination and there were other job opportunities that didn’t mandate vaccines [some employees] would leave,” he said.

American, Alaska, JetBlue and Southwest each told staff over the past week that they must be vaccinated to comply with the federal contractor rules. Previously, those airlines encouraged staff to be vaccinated but didn’t require it. United had implemented its own vaccine mandate in August and has said that more than 96% of its 67,000 U.S. employees are vaccinated after the deadline passed last week. The Chicago-based airline said it encouraged its contractors to mandate vaccines as well.

Those large airlines are federal contractors, providing services like cargo flights or tickets for government employees, among other services. Southwest does not use third-party regional airlines.

Regional airlines operate some of those flights under the City Pair Program, which sets fares for federal workers to some 12,000 markets. For example, a search for a Delta flight from Lehigh Valley International Airport in Pennsylvania to Detroit showed an option operated by Endeavor Air. A United flight from California Redwood Coast – Humboldt County Airport to Los Angeles showed one operated by SkyWest. The U.S. General Services Administration, which administers the City Pair Program, didn’t respond for a request for comment.

A recent daily schedule showed about 54% of United’s more than 4,000 departures were operated by regionals, 40% of Delta’s roughly 4,100 flights a day were operated by regional carriers that fly for Delta Connection and that they operated 52% of the flights on American’s schedule, though amounts fluctuate. Their share of overall capacity can be lower — just under 15% for United last year — because mainline carriers operate longer flights.

“A critical mass of U.S. airline flights are operated by third parties that have their own standards and own ways to do business including how they approach vaccinations,” said Henry Harteveldt, founder of travel industry consulting firm Atmosphere Research Group.

Atlanta-based Delta doesn’t currently mandate vaccines for its staff of roughly 80,000. It says more than 80% of them are vaccinated. In November, it will start charging unvaccinated employees an extra $200 per month for company health insurance. The surcharge doesn’t apply to Endeavor.

American Airlines late Wednesday told its staff of 100,000 people to upload proof of vaccination by Nov. 24 and that failing to do so would “result in termination from the company.”

That only applies to its mainline airline employees, not its regional carriers. Envoy, one of those carriers, said it is still reviewing the guidelines.

It is not clear how many of the regional airlines’ employees are vaccinated, but Harteveldt said the longer they wait “the more scrambling there has to be” to comply with a mandate.

United laid the groundwork for its vaccine mandate for months before it was announced. It also worked with unions to establish vaccine incentives, ensure they have access to the shots and policies for people who need medical exemptions.

“United worked in a very collaborative way with their unions, but it was a monthslong process,” Harteveldt said. “It wasn’t an hourlong Zoom call.”