Vaccines are best at preventing outbreaks of diseases like measles and whooping cough when immunization rates are at or above 95%. This "herd immunity" also helps protect the most vulnerable members of our communities: newborns, the very elderly, and others who can't be vaccinated for medical reasons.

But in recent decades, more families citing religious or other personal beliefs are declining to have their children immunized. In several places across the nation, childhood vaccination rates have begun to fall sharply below 95%, and vaccine-preventable diseases like measles and whooping cough are on the rise.

New York City's private schools are an acute case of this dwindling immunity. For the 2014-2015 academic year, 24 schools in the five boroughs reported vaccination rates under 75%. Another 46 institutions reported rates between 75% and 85%. While many of these schools are orthodox or conservative religious organizations, some, like the Manhattan Free School, serve up alternative education to secular elites; think comic-book making or video-game studies, instead of catechism or Torah.

Although all schools in New York City fall under the same state-level immunization requirements, private schools——particularly religious private schools——can easily facilitate vaccination exemptions. A school simply has to offer testimony that “in the opinion of the institution” the student and/or their legal guardians hold “genuine and sincere religious beliefs” that forbid immunization.

Declining herd immunity has resulted in marked outbreaks of vaccine-preventable diseases. In 2013, New York City saw 58 cases of measles, the largest outbreak in over two decades, and almost triple the size of the last major outbreak in 2008. Each of those 58 cases was located in an Orthodox Jewish community in Brooklyn. In 2015, at least 109 cases of whooping cough emerged just in the city’s Orthodox neighborhoods alone when most years see only 200 cases across all five boroughs. But although these incidents seem to suggest the problem only lies in religious communities, it’s important to remember that alternative educational institutions catering to secular, well-to-do families are also contributing to declining immunity in New York City.