Dec 20 (Reuters) - Washington, DC will reinstate an indoor mask mandate beginning Tuesday, December 21, until the morning of January 31, Mayor Muriel Bowser said on Monday. The mayor added that all employees, contractors and grantees of the District of Columbia government must be fully vaccinated against COVID-19 and must have a booster shot. There will be no test-out option, the mayor said. There was no deadline specified for the vaccination guidance.