This article previously appeared in IAC Express, a publication of the Immunization Action Coalition, and is reproduced here with the permission of the author and IAC. Dr. David Keller is a physician and former Chief of the Infectious Disease Epidemiology Program with the New Mexico Department of Health, USA. IAC is a valuable source of information on immunisation issues and produces a number of educational materials on vaccines and immunisation, which can be downloaded from their website at www.immunize.org.