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Vaccine skeptics were planning a lawsuit against New York City. A Hasidic woman was heckled when she boarded a public bus. Family members were avoiding weddings for fear of encountering unvaccinated relatives.

When Mayor Bill de Blasio announced on Tuesday an emergency health order requiring measles vaccinations, he said the step was necessary to curtail the large measles outbreak in the ultra-Orthodox Jewish community. But as health officials plunged into Brooklyn’s Williamsburg neighborhood to enforce the mandate, tensions only escalated.

Mr. de Blasio’s effort seemed to mobilize an already well-organized network of vaccine skeptics. Their latest tactic: circulating court affidavits for community members to sign so they can express opposition to mandatory vaccinations ahead of a planned lawsuit.

“I am a religious Jew, whose religious convictions are being blatantly violated by the vaccine Diktats, which are a clear violation of the Nuremberg Code, which forbids forcing medical procedures on anyone without their fully-informed consent,” the form says.