CHICAGO (AP) — Measles outbreaks across the country have prompted several Illinois health systems to pressure parents who haven’t vaccinated their children to do so.

One of the systems, Lurie Children’s Hospital, is sending letters to families of its primary care patients 16 months and older who haven’t had a first dose of the vaccine and 7 years old and older who haven’t received a second dose.

Department of Pediatrics vice chairman Dr. Matthew Davis said Lurie decided to step up its efforts to reach unvaccinated children because the current spread of measles is a reminder society depends on vaccination to keep serious and fatal diseases in check.

The Centers for Disease Control and Prevention recommends all children get two doses of measles vaccine, which it says is 97% effective.