A cross-sectional study on vaccine coverage and vaccine effectiveness was carried out on a randomized sample of the cohort of schoolchildren born in 1983 attending school in Andorra, prior to the introduction of a Systematic Immunisation Plan that included centralised import and delivery of vaccines to vaccinating clinics, surveillance of the cold-chain during vaccine delivery, and a clearly-defined immunization schedule against diphtheria, tetanus, -pertussis, polio, mumps, rubella and measles. Vaccine coverage was estimated from vaccination card records; history of disease and sociodemographic variables were obtained through a questionnaire to the children's parents and vaccine effectiveness was estimated through serum antibody testing. Vaccine coverage levels for DTP and OPV were 97.8% for both. Protective serum antibody prevalence was correspondingly high except for the polio viruses. The authors suggest that decreased vaccine effectiveness, probably due to poor preservation of the cold chain, might be the cause of this finding. In countries or regions with an otherwise developed organisation of health services, an important issue like this can still be overlooked.