All government workers in New York City must receive a coronavirus vaccine or face weekly tests as case counts from the Delta variant have surged, Mayor Bill de Blasio announced on Monday.

“The Delta variant is deadly and this city is taking it seriously,” de Blasio via Twitter post that went out simultaneously with his live announcement on television. “Today, I’m announcing that every city government employee will be required to provide proof of vaccination or submit a weekly COVID test.”

The requirement would apply to roughly 340,000 city workers, including teachers and police officers, as the city faces a third wave of coronavirus cases driven by the spread of the Delta variant.

The number of virus cases has risen to more than 800 on average per day, more than triple the daily average in late June.

Nearly five million New Yorkers have received at least one dose of a coronavirus vaccine, but the speed of inoculations has slowed. Two million adult New Yorkers are unvaccinated, according to city records.

De Blazio’s announcement came ahead of the September 13 deadline for school reopening, when a million students are set to return to classrooms in New York.