CHICAGO—U.S. airlines have come a long way since the spring of 2020 when COVID-19 brought the industry to its knees. Yet the pandemic will loom large when big carriers report quarterly earnings starting on Thursday. The latest wave of the health crisis, driven by the highly contagious Omicron coronavirus variant, has caused havoc for a short-staffed industry. A multifold increase in daily sick calls, as well as a series of winter storms, have led to mass cancellation of flights. For example, in one day alone, nearly one-third of United Airlines’ workforce in Newark, New Jersey called in sick. The Chicago-based carrier has 3,000 employees who are currently infected with the virus. Since Christmas Eve, U.S. airlines have canceled more than 30,600 flights, or about 7 percent of the scheduled total, according to flight-tracking service FlightAware – one of the biggest disruptions in recent years. Until the Omicron variant began to …