A small but growing number of parents think vaccines against childhood diseases are unsafe and are refusing or delaying shots for their children, despite the discrediting of a medical study linking vaccines and autism that stirred alarm. Ground zero in the debate is the pediatrician's office. Some frustrated pediatricians are drawing a line in the sand by requiring parents in their medical practices to vaccinate their children or seek healthcare elsewhere, a position that rubs some medical professionals the wrong way. Among those taking a stand are the eight pediatricians of Northwestern Children's Practice in Chicago. They no longer see children whose parents refuse to follow the childhood immunization schedule developed by the Centers for Disease Control and Prevention and the American Academy of Pediatrics. A letter and email sent to parents this year announced the policy, which went into effect in June.