While US infant immunization rates have been increasing in the last 20 years, the cost of fully immunizing a child with all recommended vaccines has almost tripled. This is partly not only due to new additions in the list of recommended vaccines but also due to the use of new, safer, but more expensive technologies in vaccine production and distribution. In recent years, many states have mandated that recommended childhood vaccines be covered by private health insurance companies. Currently, there are 33 states with such a mandate. In this paper, I examine whether the introduction of mandates on private insurers affected immunization rates. Using state and time variation, I find that mandates increased the immunization rate for three vaccines--the diphtheria-tetanus-pertussis, polio, and measles-mumps-rubella vaccines--by about 1.8 percentage points. These results may provide a lower bound for the expected effect of the Affordable Care Act, which mandates coverage of childhood vaccines for all private insurers in the USA. I also find evidence that the mandates shifted a significant portion of vaccinations from publicly funded sources to private ones, with a decline in public health clinic visits and an increase in vaccinations at hospitals and doctor's offices.