EFFICACY OF MUMPS IMMUNIZATION: A RETROSPECTIVE COHORT STUDY

Key words: Mumps, Immunization, Epidemiology

An outbreak of epidemic parotitis occurred in an area of Northeastern Italy from September 1995 to July 1996. To investigate the possible causes, and particularly to test the hypothesis of a primary failure due to one or more mumps vaccines, the Authors carried out a retrospective cohort study, based on a questionnaire to families of children aged 3 to 5 years. The questionnaire explored the child immunization status and time of occurrence of the disease and its results were checked with the immunization service records. On the basis of incidence density of mumps cases, vaccine efficacy was calculated for each of the 3 different vaccines that were used, controlling for age at immunization. The efficacy was particularly low for one of the vaccines (due to recall bias a "negative" efficacy was shown) and as a consequence this vaccine was excluded from subsequent immunization programs.