• United had stopped offering employees vaccines at airports late last summer.
• More than 96% of the company's roughly 67,000 U.S. employees are vaccinated, following a companywide mandate.

United Airlines started offering staff vaccines again at some of its busiest airports this week as the omicron variant continues to spread throughout the U.S. and within its own employee ranks.

The Chicago-based airline is administering Covid-19 booster shots at several of its busiest hubs: Newark Liberty International Airport, George Bush Intercontinental/Houston Airport, Chicago O'Hare International Airport and Won Pat International Airport in Guam, spokeswoman Leslie Scott said.

The airline last August established the strictest vaccine requirements of any U.S. carrier, telling staff to get immunized unless they get a religious or medical exemption, or face termination. More than 96% of United's roughly 67,000 U.S. employees have been vaccinated.

The company stopped its airport vaccine program late last summer.

"This is another step we are taking to educate our employees on the importance of boosters and make them easily accessible," said Scott. The company isn't currently changing its definition of fully vaccinated to include boosters, she said.

United's revival of the airport vaccination program comes as the rapid spread of omicron sidelined airline workers, contributing to 20,000 flight cancellations between Christmas Eve and the first week of January.

United's CEO, Scott Kirby, on Monday said that 3,000 of the airline's roughly 67,000 U.S. employees were out from Covid infections and that a third of its staff on one recent day at its hub at Newark Liberty International Airport in New Jersey had called out sick from the virus. He said the company averaged one Covid-related death per week before the vaccine mandate and that no vaccinated United employees have died from causes tied to the virus in the last eight weeks.

Delta Air Lines CEO Ed Bastian on Thursday said that about 1 in 10 of its employees had tested positive for Covid in the last four weeks but that no serious health issues have been reported.

Delta offers staff as well as employees' friends and families vaccines at its flight museum near its Atlanta headquarters, spokesman Morgan Durrant said.

Correction: Morgan Durrant is a spokesman for Delta Air Lines; Leslie Scott is a spokeswoman for United Airlines. An earlier version of this article misstated that information.