The mayors of Brockton and Revere on Monday announced they were implementing mask mandates for all city buildings.The decisions of Brockton Mayor Robert Sullivan and Revere Mayor Brian Arrigo come as the rate of new COVID-19 cases increases across much of Massachusetts. According to the most recent map from the U.S. Centers for Disease Control and Prevention, nine Massachusetts counties have “high” or “substantial” rates of transmission.Last week, the CDC issued guidance recommending that everyone “wear a mask indoors in public if you are in an area of substantial or high transmission.”On Friday, the Massachusetts Department of Public Health issued its own updated mask guidance and advised vaccinated people who have weakened immune systems, who are at an increased risk of severe COVID-19 because of age or health conditions, or who live with anyone who is unvaccinated or at increased risk, to wear a face covering when indoors and not in their own homes.”Due to the recent rise in active cases of COVID-19, Mayor Robert Sullivan has reinstated the mask mandate at all city buildings,” the City of Brockton wrote in a statement. “All employees and visitors to city buildings are required to wear a mask and practice social distancing measures.”Arrigo told NewsCenter 5 that the mask mandate for Revere city buildings will go into effect Monday.Additionally, he said that with approximately one million people coming to Revere for the return of the Sand Sculpting Festival during the upcoming weekend, he planned to assign a mobile vaccination unit to the beach.