A few decades ago, a public emergency over a measles outbreak would have seemed unthinkable. Today, however, it’s reality in New York City and so are the public-health measures needed to deal with it. Mayor Bill de Blasio’s declaration comes with a mandate for children in the epidemic zone to get vaccinated — or for the parents to start paying hefty fines:

New York City has declared the measles outbreak affecting the Orthodox Jewish community in Williamsburg to be a public health emergency, Mayor Bill de Blasio said Tuesday.

Unvaccinated people living in select ZIP codes will be required to receive the measles, mumps and rubella vaccine, known as MMR, to curtail the outbreak and protect others, he said.

Under the mandatory vaccinations, members of the city’s Department of Health and Mental Hygiene will check the vaccination records of any individual who may have been in contact with infected patients. Those who have not received the MMR vaccine or do not have evidence of immunity may be given a violation and could be fined $1,000.

It’s taken six months for the outbreak to reach the stage where an emergency declaration has become necessary. It started when an unvaccinated child got exposed to measles while on a trip to Israel, and it has spread quickly through other unvaccinated populations in New York. How did it spread so quickly? Er …

Deputy Mayor for Health and Human Services Dr. Herminia Palacio warned of “measles parties,” where parents bring together unvaccinated children with a sick child to intentionally spread the disease.

“Avoid this practice,” Palacio said. Measles parties are dangerous because the disease can be fatal. “This vaccine is safe,” she added, noting that it not only protects your child, it protects other people. “A variety of misunderstandings and frank untruths that are being propagated through a variety of channels.”

Parents did this with chicken pox before vaccinations for the disease became available. Chicken pox wasn’t deadly, however, and parents didn’t know at the time that exposure created a risk for shingles later in life. Using that strategy for measles is unconscionably risky in the short term, let alone the long term.

NYC health commissioner Oxiris Barbot explained that 285 cases in six months is enough. Excluding unvaccinated children from schools hasn’t been enough to slow the spread, which is now why the city is abandoning exceptions for religious objections. That could impose a thousand-dollar fine per child per family, even if they’re trying to avoid “vaccine injuries,” as Alisyn Camerota notes:

Give Camerota credit for making a valid point about parents’ concerns on vaccines and not treating them as nuts for having them, but Sanjay Gupta is correct here. No one wants their child to be the “one in a million” that suffers an adverse outcome from a vaccine, but the risk of the disease is far higher, especially in an epidemic situation. Maybe the risk calculation when the disease isn’t present tilts against vaccination (although that seems hard to fathom), but when measles is spreading through the community, that risk calculation has to change.

The risk calculations have to change for public health officials and communities, too. If we want to prevent these outbreaks from occurring, then cities and states will have to narrow the exceptions to vaccination requirements, and do better at educating people about vaccine safety. Otherwise, we will find ourselves in epidemics, injuries, and deaths from diseases we had beaten — not because the vaccines became ineffective, but because our judgment did.