A national day care center operator says it's requiring staff members working with children under 15 months old to be vaccinated against measles. 

KinderCare Learning Centers noted the requirement in a letter on its website dated Thursday, the same day health officials announced measles diagnoses in five infants who attended a suburban Chicago KinderCare. The cases come amid growing concern about unvaccinated children after an outbreak connected to Disneyland sickened dozens of people.

Children under age 1 generally are considered too young for vaccinations, leaving them vulnerable to contracting the disease from another unvaccinated and infected person.

KinderCare's letter also says the operator is collecting vaccination records for all staff members, and limiting access to infant rooms to either adults dropping off or picking up an infant, or immunized staff.