All teachers in California will have to be vaccinated against Covid-19 or submit to weekly virus tests, the state's governor announced Wednesday, as authorities grapple with exploding infection rates.

The number of people testing positive for the disease has surged across the United States in recent weeks, with the highly infectious Delta variant blamed for the bulk of new cases.

That has worried parents and educators as the most populous state in the country readies to send its largely unvaccinated children back into classrooms for the new school year.

"To give parents confidence that their children are safe as schools return to full, in-person learning, we are urging all school staff to get vaccinated," Gavin Newsom said.

"Vaccinations are how we will end this pandemic. As a father, I look forward to the start of the school year and seeing all California kids back in the classroom."

As it was for many children around the world, last school year in California was badly interrupted, with classes moved online, and many children without adequate internet connections missing out on huge chunks of their education.

Along with the rest of the country, the state managed to tame the worst of its coronavirus outbreak earlier this year and life is largely back to normal.

But the return to in-person learning for this academic year has been imperiled by growing infections; more than 10,000 new cases are being recorded every day in the state -- a ten-fold increase over two months.

Doctors say these infections are chiefly among the unvaccinated.

Vaccinations are free and widely available in the United States.

Losing patience

The order announced Wednesday -- the first such state-wide mandate in the country -- applies to public and private schools, and puts the onus on administrators to ensure either that staff are fully vaccinated, or that they undergo Covid-19 tests at least once a week.

Parent groups welcomed the move.

"We want to do everything possible to protect our most vulnerable children and ensure that all children can return to school as safely as possible," said California State PTA President Carol Green.

"We stand by our position that educators are essential workers and support the safe opening of schools to in-person instruction."

Around two-thirds of Californians over the age of 12 have been fully vaccinated, according to the state's health department.

Children under that age are not eligible for the shots.

Vaccinations are a divisive issue in the United States, largely along party-political lines, with the left overwhelmingly in favor and sections of the right opposed.

Take up of the vaccines varies widely across the country, and is particularly poor in the South, but there are growing signs that government and business is running out of patience with vaccine hold-outs.

The Pentagon this week announced it would make vaccines mandatory for all service personnel by next month.

New York City will soon require proof of vaccination for people attending indoor venues like gyms and restaurants, while Los Angeles appears to be moving the same way.

California has already told all of its public healthcare workers that they must be innoculated.

Major employers like United Airlines, investment bank Morgan Stanley, meat producer Tyson Foods and tech giants like Microsoft have all demanded employees get a shot.

Italy this month said teachers had to have a Green Pass, which shows either that they have been vaccinated, have tested negative for coronavirus or have recovered from a bout.