New York City health officials issued summonses to parents of three children Thursday for failing to have their children vaccinated 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 yet vaccinated by April 12, in violation of the order. Failing to appear at the hearing or respond to the summons will result in a $2,000 fine but city authorities have said they will not face criminal prosecution. The children are in three separate households.

Public health legal experts said it’s been at least a century since health authorities issued fines in connection with such violations.

Last week, the city issued the order mandating vaccinations in four hard-hit Zip codes in Brooklyn’s Williamsburg section, home to tens of thousands of ultra-Orthodox Jews and the center of a measles outbreak. Health officials confirmed 359 cases as of Wednesday— an increase of 30 cases in just two days.

New York’s Health Commissioner Oxiris Barbot said Thursday that health authorities “did not take the emergency order lightly. It was a dramatic response to a serious problem.” She also said that they would prefer not to issue violations, urging New Yorkers to get vaccinated.

Because measles’ incubation period is long — 7 to 21 days — officials said they expect the outbreak to get worse before it gets better, especially as the week-long Passover holiday approaches and families gather at one another’s homes. Outside New York city, measles cases are now reported in Orange (20), Rockland (90), Sullivan (2), and Westchester (10) counties, according to the state health department.

As part of the city’s 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.

In a related development, a Brooklyn judge on Thursday denied the petition of a group of five anonymous Brooklyn parents who had sought to stop the city order, arguing there was insufficient evidence of a dangerous enough outbreak to justify such extraordinary measures. The judge sided with city health authorities, calling the order a rare but necessary step to contain the worst measles outbreak to hit the city since 1991.

The emergency measure is the broadest vaccination order in the United States in nearly three decades. The United States 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 City Mayor Bill de Blasio (D) has said the city has sweeping emergency authority for such orders during a public health emergency.

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

Earlier in the week, officials closed a Williamsburg child care program for repeatedly failing to provide access to medical and attendance records in violation of the emergency order. The order requires child care programs to exclude unvaccinated students and staff and to maintain medical and attendance records on site. That child care program reopened Thursday.

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