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In 2019, Rockland County, New York declared a state of emergency. Measles—a disease declared eliminated in the US in 2000—had roared back. The outbreak began in October 2018 when unvaccinated travelers returned from Israel, where a large outbreak was underway. Within months, 312 cases erupted across Brooklyn and Rockland County. The math was unforgiving. Measles is one of the most contagious diseases known: each infected person spreads it to 12-18 others in an unvaccinated population. Herd immunity requires 95% vaccination coverage. In certain Rockland County zip codes, MMR vaccination rates had dropped to 77%. In some Brooklyn neighborhoods, rates sat at 85%. These numbers sound high—but the virus doesn't care about impressions. At 95% coverage, chains of transmission break. At 94%, th...
Popular framing: Anti-vax parents are stupid and selfish.
Structural analysis: Herd immunity is a tipping-point public good: dropping coverage from 95% to 90% doesn't double risk, it multiplies it tenfold, because transmission chains either break or sustain. Each family's choice has negligible private cost and a negative externality that falls on people who cannot be vaccinated — a tragedy-of-the-commons structure where the protective stock collapses long before most participants notice their individual contribution mattered.
Treating this as an information problem misses that the tragedy of the commons dynamic makes rational free-riding on others' vaccination individually optimal precisely when herd immunity is working. Without understanding the nonlinear threshold and the externality structure, interventions focus on persuasion while ignoring the enforcement and monitoring mechanisms needed to prevent commons collapse — and they arrive reactively, after the tipping point has already been crossed.
