AURORA, Colo., Jan. 30 (UPI) -- Physicians' offices find reminding patients to update their immunizations challenging despite programs to help, U.S. researchers say.

Researchers led by Alison Saville and Dr. Allison Kempe, both of the University of Colorado School of Medicine in Aurora, learned physicians in Colorado say physicians' practices under-utilize a statewide immunization registry. The Colorado Immunization Information System offers a Web-based tool listing children behind on immunizations and generates mailing labels or phone numbers.

"This is the first study to explore the real world issues that influence private practices' decisions to implement an evidence-based intervention to increase immunizations," Saville says in a statement.

The study, published in Academic Pediatrics, finds among the reasons for resistance to the system are a lack of awareness, distrust in data accuracy, and perceived difficulties with implementation or effectiveness.

Saville, Kempe and colleagues interviewed 24 physicians and staff members in 11 practices who received 2-hour, computer-based training that provided an opportunity for attendees to run real-time reminder programs with state staff assistance. Of the 11 practices, four implemented programs. Three practices focused on children under the age of 2, and one focused on adolescent females and the human papillomavirus vaccine.