As many as 40% of U.S. airport security screeners haven’t been vaccinated for COVID-19 as an immunization deadline for federal employees and the busy holiday travel season converge.

Many Transportation Security Administration workers are resisting the requirement as the Nov. 22 deadline approaches, said Hydrick Thomas, president of the American Federation of Government Employees’ division representing front-line airport security officers.

While neither Thomas nor the agency foresee any travel disruptions occurring around Thanksgiving, which at Nov. 25 is just three days later, the union chief said there could be staffing shortages during the December holidays if the agency takes a hard line on unvaccinated workers.

“They are not going to be ready for Christmas if they get rid of everybody who chooses not to get vaccinated,” Thomas said. “If they don’t accommodate employees during these holidays coming upon us, we are going to have an issue with the screening process.”

He estimated those who are hesitant represent about four in 10, though acknowledged he didn’t have hard numbers.

The prospect of large numbers of unvaccinated workers raises multiple questions, including whether they increase the risks of transmitting the disease given their close encounters with many travelers every day. The TSA requires workers to wear masks and take other precautions against infection. The issue also reflects broader concerns in the aviation industry as some pilot unions are objecting to vaccine requirements and exemption policies they see as too limited.

The TSA, whose screeners have been forced to work in close quarters with the public during the pandemic, has had 32 employees die as a result of Covid-19, according to its website. There are 271 active coronavirus cases among agency employees and more than 11,000 employees have been infecting during the pandemic.

The TSA, which has just completed a hiring surge to keep up with the gradual increases in post-pandemic flying and doesn’t anticipate any staffing losses though the holiday, predicts there won’t be any immediate disruptions as a result of President Joe Biden’s vaccination mandate.

“I don’t think implementation of the mandate will have any effect on staffing whatsoever on Thanksgiving,” TSA Administrator David Pekoske told ABC’s “Good Morning America” today.

The agency hasn’t released data on staff vaccinations, but Pekoske said it had seen “quite a significant increase.”

TSA spokesman Carter Langston declined to comment directly on Thomas’ estimates, but said the agency has made progress persuading employees to get the shot after estimating in early October that 40% of employees hadn’t done so.

“We have made significant strides since that point,” Langston said. “I can’t comment on anecdotes, but the compliance rate is very high and we don’t have the full data yet.”

The agency views the approaching deadline on Monday “not so much a cliff, but a step to further counsel any unvaccinated employees,” he said.

On the issue of whether there could be problems during the second half of December, Langston said: “It’s too early to get into any kind of projections on a holiday that’s a month out. We’re ready for this holiday travel season. We’re staffed and prepared.”

Airlines for America, which represents large U.S. carriers, said ticket sales as of Nov. 7 for travel during the Thanksgiving holiday were running 12% below the equivalent date in 2019 before the pandemic. That would be higher than at any point since March 2020, but is still well below normal levels.

TSA said in a press release today it doesn’t expect travel around Thanksgiving to reach the record numbers of 2019, but it will be “notably higher” than current levels.

Federal guidelines say that federal employees who refuse to get vaccinated and don’t qualify for a religious or medical exemption may be fired, but such action won’t occur immediately. Agencies have been instructed to first conduct education and counseling. That makes firings of TSA workers unlikely during the holiday period.

“We know that we’ve lost too many TSA employees to Covid-19 and that vaccines are the best way to protect our workforce and our communities,” the TSA’s Langston said.