A Brooklyn judge on Thursday rejected the petition from five anonymous anti-vaccine mothers who attempted to block the city’s recent vaccination mandate amid the largest measles outbreak the city has seen in several decades.

And the city wasted no time enforcing its upheld order. As the judge made his decision Thursday, city health officials doled out the first penalties to violators , according to the New York Times. Officials sent summonses to the parents of three children for failing to vaccinate the children even after city officials determined that they had been exposed to the dangerous viral illness.

Measles is so contagious that up to 90 percent of unvaccinated or otherwise susceptible individuals who are exposed will become ill, according to the Centers for Disease Control and Prevention. Measles’ extreme contagiousness is due in part to the fact that once it is launched into the air from a cough or sneeze it can remain airborne and infectious for up to two hours. Any vulnerable passersby who breathe in the virus or touch contaminated surfaces can pick it up.

With continued exposures of unvaccinated children—such as the three children of the penalized parents—health officials in the city expect that the outbreak will continue to get worse. As of April 18, officials have confirmed 359 cases in the outbreak, which started last October. That tally is up from 285 cases confirmed just ten days ago.

The parents of the three children—who are from three separate households—face $1,000 fines if an officer upholds the summons in a hearing. If the parents fail to respond to the summons or appear at their hearing, they will be fined $2,000.

New York City Health Commissioner Oxiris Barbot said Thursday that officials did not make the decision to issue the mandate—and its associated penalties—lightly. “It was a dramatic response to a serious problem,” she said, according to the Times.

Earlier this week, lawyers for the health department vigorously defended the mandate in court. Based on the lawsuit (PDF) brought by the anonymous mothers, their attorney claimed that the order is “excessive, it’s coercive.” The city’s lawyers countered that the pace of the outbreak’s spread has quickened and remains a serious public health concern. They also noted that the mothers’ vaccination objections relied on bogus and discredited science.

The judge in the case, Judge Lawrence Knipel, sided with the city, calling it a rare but necessary response to a dangerous outbreak.