New York health officials issued summonses to parents of three children Thursday for not vaccinating their children against measles, a violation of the city’s emergency order mandating immunizations to control a surging outbreak.

The adults face civil penalties of $1,000 if an officer upholds the summons at a hearing. Health officials identified three children who were exposed to the severe respiratory virus but were not vaccinated by Friday, violating the order. Skipping the hearing or not respond to the summons will result in a $2,000 fine. The children are in three households.

During the outbreak, the city has mandated vaccinations for people living in four hard-hit Zip codes in Brooklyn's Williamsburg section, home to tens of thousands of ultra-Orthodox Jews. Health officials also confirmed 359 cases as of Wednesday, an increase of 30 cases in two days.

Because measles’ incubation period is long — seven to 21 days — health officials said they expect the outbreak to get worse before it gets better. “However, we can turn the tide by people getting vaccinated, especially before Passover, when families and communities gather,” said New York’s Health Commissioner Oxiris Barbot.

As part of the Health Department investigation, disease detectives trace where each infected person went while contagious and everyone they may have interacted with. If investigators find an unvaccinated child who could have been exposed to measles, that child's parents could be subject to fines, officials have said.

During a meeting Wednesday, the city's health board said it would be seeking civil, not criminal, penalties against the parents. Earlier in the week, a group of Brooklyn parents filed a lawsuit in Brooklyn Supreme Court, seeking to stop the city order. The plaintiffs say the order is unlawful because there is insufficient evidence of a measles epidemic or a dangerous enough outbreak to justify extraordinary measures, such as forced vaccination or criminal penalties.

The measure is the broadest vaccination order in the United States in nearly three decades. The nation has a long history of mandatory vaccinations dating back to a 1905 Supreme Court case that upheld Massachusetts' authority to require vaccination against smallpox during an epidemic after a man defied an order to be vaccinated. The case laid the foundation for public health laws in the United States.

New York Mayor Bill de Blasio, a Democrat, has said the city has sweeping emergency authority for such orders during a public health emergency.

Health officials also announced Thursday that they closed four additional schools for noncompliance with measles vaccine requirements.

Earlier in the week, officials closed a Williamsburg child care program for repeatedly not providing access to medical and attendance records, violating the emergency order. The order requires child care programs to exclude unvaccinated students and staffers and to maintain medical and attendance records on site. That child care program reopened Thursday.