In response to the outbreak, schools in Washington, New York and Los Angeles have urgently adjusted their back-to-school arrangements after the New Year holiday, requiring students to show negative nucleic acid tests, and many schools have temporarily switched to online classes. Maryland Governor Larry Hogan declared a 30-day state of emergency on January 4 and deployed 1,000 National Guard troops to help fight the disease. U. S. President Joe Biden on Thursday urged adults who have not yet been vaccinated to get vaccinated. The U. S. Centers for Disease Control and Prevention also updated its guidelines to recommend a five-day quarantine if people show symptoms of suspected infection or test positive. However, the guidelines also include shortening the recommended quarantine period for asymptomatic carriers to ease the labor shortage in the airline and other services, which has raised doubts among medical experts.