2, 2010, to combine operations, contingent upon shareholder and regulatory approval. On October 1, 2010, the UAL Corporation changed its name to United Continental Holdings, Inc. The carriers planned to begin merging their operations in 2011. The merged airline began operating under a single air operator's certificate from the Federal Aviation Administration on November 30, 2011. On March 3, 2012, United and Continental merged their passenger service systems, frequent-flier programs, and websites, which virtually eliminated the Continental brand with the exception of its logo. On June 27, 2019, the parent company's name changed from United Continental Holdings to United Airlines Holdings. In January 2021, Chief Executive Scott Kirby put forward the possibility for the company to mandate employees to receive the COVID-19 vaccine while cautioning the potential difficulties in implementing the mandate. The company was the first major US airline to announce a vaccine mandate for all staff on Aug 6, at which point over 80% of flight attendants and 90% of pilots had been vaccinated, according to statements of the respective unions. Days before the internal deadline of Sept 27, the company announced that more than 97% of the US based staff were vaccinated. Destinations and hubs United operates flights to 238 domestic destinations and 118 international destinations in 48 countries or regions across five continents. By spring 2021, United will once again fly regularly to all 6 inhabited continents following the reinstatement of scheduled year-round flights to Africa. Hubs As part of its hub-and-spoke business model, United currently operates eight hubs. Chicago–O'Hare – United's hub for the Midwest and largest hub overall. United controls 47% of the market share in O'Hare, making it the largest carrier at the airport. United's corporate headquarters are also in Chicago. Denver – United's hub for the Rocky Mountain region of the United States. United has about 42% of the market share in Denver, making it the largest carrier at the airport. By number of flights it has become United's largest hub in 2021. Guam – United's hub for flight routes in the Pacific region. United has about 98.8% of the market share at Guam International, making it the largest carrier at the airport. Houston–Intercontinental – United's hub for the Southern United States and primary gateway to Latin America. United currently has about 78% of the seat share at IAH, making it the largest carrier at the airport. Former Continental Airlines hub. Los