A New York county has banned unvaccinated children from all public spaces as the state battles its largest measles outbreak in decades.

Officials in Rockland County declared a countywide state of emergency Tuesday, announcing that the ban will begin at midnight and remain in place for 30 days or until unvaccinated minors receive the measles, mumps and rubella (MMR) vaccine.

Officials said unvaccinated minors will not be permitted in public places, such as churches, schools and shopping centers, though outdoor spaces such as playgrounds are not included in the ban.

"We must not allow this outbreak to continue," County Executive Ed Day said during a news conference. "We will not sit idly by while children in our community are at risk."

The announcement comes as measles outbreaks have hit areas in California, Illinois, Texas and Washington, according to the Centers for Disease Control and Prevention.

In New York City, more than 180 cases have been confirmed.

In Rockland County, about 40 miles (65 kilometres) from Manhattan, there have been more than 150 confirmed cases, according to county health officials.

More than 82 percent of the measles patients had not received a single dose of the MMR vaccine, health officials said. The data shows that the largest number of cases - 46 percent - were seen in children ages 4 to 18, and 39 percent of them were in children younger than 3.

Day, with Rockland County, said authorities will not be searching for children who are not vaccinated but are expecting parents and legal guardians to step up and get children vaccinated. However, he said, parents and guardians who are found to be in violation will be held accountable and their cases will be referred to the district attorney's office. Such a violation will be considered a misdemeanor, punishable by a US$500 fine or up to six months in jail.

Day said children unable to be vaccinated for documented medical reasons are exempt.

"Rockland will lead the way in service and safety to the people here," he told reporters.

Amid concerns about the growing measles outbreak, Rockland County tried something similar last year.

As The Washington Post's Reis Thebault reported, public health officials there barred unvaccinated children from attending schools with vaccination rates lower than 95 percent.

He added:

Months later, the parents of more than 40 banned children at Green Meadow Waldorf School sued the Rockland County health department, asking a federal judge to allow the students to return to class. This week, US District Court Judge Vincent Briccetti denied their request, ruling it wasn't in "public interest" to allow the children to go back to school. "While no one enjoys the fact that these kids are out of school, these orders have worked," said the county's attorney, Thomas Humbach, in a statement to the local Journal News. "They have helped prevent the measles outbreak from spreading to this school population."

Measles is a highly contagious virus and can have serious consequences - pneumonia, brain damage, hearing loss and even death, according to the CDC.

Before the introduction of the measles vaccine in 1963, most children contracted the illness - an estimated 3 million to 4 million patients were infected each year in the United States, according to CDC data.

Of those, 48,000 were hospitalized, 400 to 500 died and 1,000 others suffered a severe complication known as encephalitis, a condition in which the brain swells because of an infection.

In 2000 - almost four decades after parents began vaccinating their children - measles was declared eliminated in the United States.

CDC data shows that from 2000 to 2018, there were an average of 140 measles cases per year in the United States. And there were three reported fatalities during that time - one in 2002, one in 2003 and one in 2015.

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