CHICAGO—United Airlines Chief Executive Scott Kirby said on Wednesday the company’s decision to fire employees who defied its vaccine mandate will not affect the carrier’s operations. In an interview to CNBC, Kirby also said ticket sales have bottomed out, with the bookings for lucrative business travel rebounding to the levels seen in June. The comments came a day after the Chicago-based airline said it would start the process of firing 593 employees who failed to comply with its COVID-19 vaccination policy. In early August, United became the first U.S. carrier to require vaccinations for all domestic employees, requiring proof of vaccination by Monday. The company had plans to put employees who received religious exemptions on temporary, unpaid personal leave starting Oct. 2. Those plans, however, have been put on hold until Oct. 15 because of a lawsuit challenging the policy. Six United employees filed a class action in federal court …