The 737 MAX returned to service in November after a 20-month safety ban triggered by two fatal crashes that killed 346 people. The plane has continued to face careful scrutiny since it resumed flights.
Southwest Airlines removed 30 MAX airplanes from its schedule Friday, while American Airlines removed 17 of its 41 MAX airplanes and United Airlines removed 16 of its 30 MAX airplanes.
Earlier, Southwest Airlines and JetBlue Airways also pointed to a steady rise in leisure bookings as signs of a slowing pandemic due to vaccine rollouts drive more people to go on vacations or visit their friends and relatives.
The US vaccine roll-out has been patchy and many European countries are discouraging travel and implementing more travel curbs to contain the spread of new infections.
Despite strong cost-cutting efforts in place, airlines continue to burn millions of dollars of cash every day.
Budget carriers Spirit Airlines, Allegiant Travel and Frontier Airlines, however, have said in recent weeks that they intend to resume pilot hiring later this year.
Airlines are seeking at least 14 days before new requirements take effect and "consideration of inadequate testing and results availability in specific countries rather than a blanket worldwide requirement is also needed," the letter said.
The Dow Jones Industrial Average added 0.2 percent at 30,046.37, while the broad-based S&P 500 declined 0.1 percent and the tech-rich Nasdaq Composite Index shed 0.3 percent.
For the vaccine rollout, airlines said they expect some direct shipper arrangements with the manufacturers themselves, and enough business for any airline with the capacity and infrastructure to handle it.