New York City health officials are threatening to fine or shut down certain Brooklyn yeshivas that allow unvaccinated children to attend school, as a measles outbreak continues to spread within the city’s Orthodox Jewish community.

In December, the Health Department ordered yeshivas and childcare centers in parts of Brooklyn’s Borough Park and Williamsburg neighborhoods to keep children home from school if they had not received the full course of the measles, mumps and rubella (MMR) vaccine. But at least one yeshiva, Yeshiva Kehilath Yakov in Williamsburg, broke the city’s mandate in January and allowed an unvaccinated, but asymptomatic, student to return to school. The student turned out to have measles, resulting in more than 40 cases connected to this yeshiva alone, according to health officials.

On Tuesday, the Health Department announced that Williamsburg yeshivas that do not follow the exclusion order will “immediately be issued a violation,” and could face fines or possible closure.

Almost 300 people in New York City have contracted measles since the city’s outbreak began in October, according to health officials. The vast majority of cases have affected children, particularly in Borough Park and Williamsburg. The outbreak’s initial case was acquired when a person visited Israel, where a measles outbreak is in progress, according to health officials.

“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,” Health Commissioner Dr. Oxiris Barbot said in the new statement. “We stand with the majority of people in this community who have worked hard to protect their children and those at risk. We’ve seen a large increase in the number of people vaccinated in these neighborhoods, but as Passover approaches, we need to do all we can to ensure more people get the vaccine.”

Measles is a viral infection that causes symptoms including fever, runny nose, cough and a rash that affects the face and body. In severe cases, measles complications can include pneumonia, brain swelling and death. It’s highly contagious and is spread through the air. No one has died from New York City’s measles outbreak, through 21 people have been hospitalized, according to city officials.

Measles was declared eliminated from the U.S. in 2000, but the disease is having a resurgence in several parts of the country — particularly in areas where vaccine resistance is common, including some pockets of the Pacific Northwest. So far this year, the Centers for Disease Control and Prevention has confirmed 465 cases of measles, already the second-highest number since 2000.

Communities experiencing outbreaks are taking increasingly dramatic measures to stop transmission of the disease. In Rockland County, N.Y., for one, officials banned unvaccinated children from visiting public places, until a judge halted the order last week.

Write to Jamie Ducharme at jamie.ducharme@time.com.