The mayor of New York City announced Monday that teachers, police officers and the rest of the city's 340,000 employees must be vaccinated by mid-September or face weekly coronavirus testing, while California announced a vaccine-verification program for all state and healthcare employees, and the federal Veterans Administration issued a vaccine mandate for healthcare workers.

"This is about our recovery. This is about what we need to do to bring back New York City," Mayor Bill de Blasio said. "This is about keeping people safe."

Renee Campion, commissioner at the New York City Office of Labor Relations, said if employees refuse to comply, they will be put on leave without pay.

Monday's decision makes New York City one of the largest employers in the US to take such action. The mayor said the announcement was part of what will be an intense vaccination effort leading up to the start of the school year. The New York City mandate begins after Labor Day.

"September is when many employers are bringing back a lot of their employees," he said. "September is when school starts full strength. September is when people come back from the summer."

Hours after New York City's announcement, officials in California announced that state employees and all healthcare workers will be required to show proof of COVID-19 vaccination or get tested weekly.

Starting in August, approximately 238,000 state employees in California, along with healthcare workers, must show proof of their vaccination status or be tested regularly, Governor Gavin Newsom said Monday.

Those who can't verify they are fully vaccinated must wear masks in the office and be tested for the virus weekly, state health officials said.

"Too many people have chosen to live with this virus," Newsom said at a news conference Monday. "We're at a point in this pandemic where individuals' choice not to get vaccinated is now impacting the rest of us, and in a profound and devastating and deadly way."

Newsom and de Blasio, both Democrats, also urged private employers to implement similar vaccination mandates.

The Veterans Affairs Department on Monday became the first federal agency to issue a vaccine mandate, requiring all of its medical employees to get a jab within eight weeks. It said that four of its employees, all of whom were unvaccinated, had died from COVID-19 in recent weeks.

"Whenever a veteran or VA employee sets foot in a VA facility, they deserve to know that we have done everything in our power to protect them from COVID-19," Veterans Affairs Secretary Denis McDonough said in a statement.

Meanwhile, medical groups representing millions of doctors, nurses, pharmacists and other health workers on Monday called for mandatory vaccinations of all US health personnel against the coronavirus.

"We call for all healthcare and long-term care employers to require their employees to be vaccinated against COVID-19," the American Medical Association, the American Nurses Association and 55 other groups wrote in a joint statement, The Washington Post reported. "The health and safety of US workers, families, communities, and the nation depends on it."

It was the first time the groups issued a statement to urge a mandate, representing an increasing tough stance by the medical and public health establishment amid the sluggish pace of national vaccinations.

At the same time, federal regulators have requested vaccine companies Pfizer and Moderna to expand the size of their clinical trials for children ages 5 to 11, which began in late March.

The move is to assess whether a rare inflammation of the heart muscle that has been seen in young adults shortly after vaccination is more common in younger age groups.

The New York Times reported Monday that the FDA asked both Moderna and Pfizer to include 3,000 children in the 5- to 11-year-old trials, citing anonymous sources.

The changes could delay the availability of the vaccines to children between the ages of 5 and 11, which was scheduled for early fall, The Washington Post reported.

The FDA is likely to require two months of follow-up data for the 5-to-11 age group, as it did for adults and adolescents. For children under 5, the agency may require four to six months of follow-up data, meaning vaccines are unlikely to be available for several months for infants and young children, according to the Post.

Meanwhile, several cities, including Savannah, Georgia; St. Louis, Missouri; and Provincetown, Massachusetts, reinstated indoor mask mandates to stop the spread of COVID-19.

St. Louis will require those age 5 and older to wear masks in indoor public spaces and on public transportation. Mayor Tishaura Jones said the city is still at a "dangerous point" with the virus, facing infection numbers not seen since December.

State Attorney General Eric Schmitt, a Republican who is also running for US Senate, threatened to file a lawsuit to prevent the mandate. In a tweet, he referred to the measure as "insane" and positioned the lawsuit as an effort to "protect freedom".