National Network for Immunization Information: http://www.immunizationinfo.org/
WHO - immunization: http://www.who.int/topics/immunization/en/
American Academy of Pediatrics - Immunization: http://www.aap.org/healthtopics/Immunizations.cfm
Infectious Diseases Society of America: http://www.idsociety.org/
Centers for Disease Control and Prevention: http://www.cdc.gov/