New York City officials declared a public health emergency Tuesday in parts of the Williamsburg neighborhood, where a measles outbreak has run rampant in an Orthodox Jewish community since last October—largely among unvaccinated children.

As part of the emergency declaration, any unvaccinated people living in affected zip codes who may have been exposed to the highly contagious viral illness will now be required to receive the Measles, Mumps, and Rubella (MMR) vaccine or prove immunity. Violators could face a fine of $1,000, officials said.

So far, there have been 285 cases of measles in the outbreak, 246 of which were in children. Most of the children were unvaccinated or partially vaccinated. The cases led to 21 hospitalizations, including five admissions to intensive care units.

New York City is just one of seven locations around the country currently experiencing a measles outbreak. The other locations include areas of Washington state and Michigan, Butte County and Santa Cruz County in California, Rockland County in New York, and Ocean County in New Jersey. Since the start of the year, the Centers for Disease Control and Prevention has tallied 465 measles cases in 19 states. That tally from just over three months in 2019 exceeds the total number of cases from all of 2018, which was 372.

Health officials have largely blamed the outbreaks on small but vocal anti-vaccine advocates who scare well-meaning parents by spreading dangerous lies and misinformation about life-saving vaccines and their safety. Officials in New York City were no different in their blame.

“There’s no question that vaccines are safe, effective, and life-saving,” New York City Mayor Bill de Blasio said in a statement Tuesday. “I urge everyone, especially those in affected areas, to get their MMR vaccines to protect their children, families and communities.”

City Health Commissioner Dr. Oxiris Barbot added, “As a pediatrician, I know the MMR vaccine is safe and effective. This outbreak is being fueled by a small group of anti-vaxxers in these neighborhoods. They have been spreading dangerous misinformation based on fake science. We stand with the majority of people in this community who have worked hard to protect their children and those at risk.”

On Monday, the city’s health department also issued an order requiring all Orthodox Jewish schools in the area to exclude unvaccinated children until the outbreak is over. Schools that don’t comply face fines and closure.

New York City isn’t the only area sounding the alarm and taking a firm stance on vaccination amid outbreaks. Officials in Washington and in New York’s Rockland County also issued emergencies. Rockland took the unprecedented step of banning unvaccinated children from public spaces to curb the spread of disease. However, a state judge on Friday temporarily halted the ban, and health officials are said to be working on other strategies.