New York announced a vaccine mandate for all public school staff, including teachers and principals, Monday as the city escalates attempts to slow the spread of the hyper-contagious Delta variant.

Mayor Bill de Blasio said every department of education employee must receive at least one dose of a coronavirus vaccine by September 27.

There will be no option of a weekly test instead.

"We know this is going to help to ensure that everyone is safe," he told reporters.

New York City has America's largest public school system with some 1.1 million students across 1,800 schools. The directive will affect almost 150,000 employees.

Just 63 percent of education staff are vaccinated so far.

The announcement comes as public bodies and private businesses in the United States grapple with boosting vaccination rates.

Many unions and critics of mandates have spoken out against required vaccinations, citing personal freedom arguments.

On Monday the US Food and Drug Administration fully approved the Pfizer-BioNTech Covid vaccine, which is expected to trigger a slew of new vaccine mandates around the country.

New York's directive comes after Los Angeles and Chicago announced their own vaccine mandates for teachers.

De Blasio wants all students to return to classrooms when schools start the new academic year on September 13 after a mixture of remote and in-person learning last year.

The Big Apple now requires proof of vaccination for people attending indoor venues such as restaurants, gyms and shows, as the city tries to get back on its feet.

In New York, a city of more than eight million inhabitants, 75.6 percent of adults have received at least one dose of vaccine, according to official figures.