WASHINGTON, Jan 11 (Reuters) - United Airlines said on Tuesday that about 4% of its U.S. employees, or about 3,000 workers, have tested positive for COVID-19 but vaccinated employees had neither died nor recently been hospitalized with the disease. United"s disclosure comes amid soaring cases and continuing debate about U.S. vaccine mandates. On Friday, the U.S. Supreme Court heard arguments to challenges to President Joe Biden"s vaccine or testing mandate for companies with more than 100 employees. COVID-19 cases and quarantines are impacting staffing and hitting flight schedules. U.S. airlines have cancelled more than 30,000 flights since late December over weather and COVID-19 staffing issues. United Airlines Chief Executive Scott Kirby said on Tuesday the airline was "reducing our near-term schedules to make sure we have the staffing and resources to take care of customers." United canceled 149 flights on Tuesday, or 7% of its flights, according to FlightAware.com. Chicago-based United was the first U.S. carrier to mandate vaccines for its employees in order to facilitate travel and flight operations. Kirby said "the Omicron surge has put a strain on our operation ... Just as an example, in one day alone at Newark, nearly one-third of our workforce called out sick." Kirby said in a memo on Tuesday that "since our vaccine policy went into effect, the hospitalization rate among our employees has been 100x lower than the general population in the U.S." Before United"s vaccination requirements were put in place, "tragically, more than one United employee on average *per week* was dying from COVID," Kirby said. "But we’ve now gone eight straight weeks with zero COVID-related deaths among our vaccinated employees." He added: "While we have about 3,000 employees who are currently positive for COVID, zero of our vaccinated employees are currently hospitalized." United’s prior experience and nationwide COVID data suggests "there are approximately 8-10 United employees who are alive today because of our vaccine requirement," said Kirby. Last month, Kirby defended the airline"s vaccine mandate decision in the face of Republican criticism. "We did this for safety," Kirby said at a U.S. Senate hearing. "We don"t compromise on safety. Kirby said last month about 200 employees did not comply with United"s mandate and were fired out of its 67,000 employees. Rival American Airlines (AAL.O) said on Friday that more than 96% of its employees have submitted proof of COVID-19 vaccination or a request for an accommodation. Last month, Delta Air Lines (DAL.N) asked the Centers for Disease Control and Prevention to soften quarantine guidelines for fully vaccinated individuals who experience breakthrough COVID-19 infections, warning the long quarantines may "significantly impact our workforce and operations." Other airlines followed Delta"s request. The CDC last month agreed to shorten the recommended isolation time to five days from the previous guidance of 10 days.