MEASLES OUTBREAK IN WESTERN SYDNEY - VACCINE FAILURE OR FAILURE TO VACCINATE

Objectives: To determine the effectiveness of measles vaccine during a measles outbreak, and to assess whether age at vaccination was a risk factor for measles vaccine failure. Design: A matched case-control study. Setting: The five primary schools in western Sydney with the largest number of measles cases during the June to December 1993 outbreak. Subjects: Seventy-nine children aged 5-9 years with an illness consistent with a clinical definition for measles. Two controls per case were selected from children in the same classroom. Main outcome measures: Estimated measles vaccine effectiveness by age of the child at vaccination and vaccination status: ''unvaccinated''; ''parental recall'' (parents stated the child was vaccinated but no record could be found); and ''record'' (record including date of vaccination available). Results: The estimated vaccine effectiveness was 94% (95% confidence interval [CI], 83%-98%) in the ''record'' group, and 81% (95% CI, 46%-93%) in the ''parental recall'' group. Vaccine effectiveness did not differ significantly with age at vaccination (under 12 months of age 96% [64%-99%]; 12-14 months 95% [81%-99%]; and 15 months and over 93% [80%-98%]). Conclusion: Vaccination records should be used to calculate a vaccine's effectiveness as parental recall may not be sufficiently accurate. The high vaccine effectiveness in the ''record'' group (94%) makes it unlikely that low vaccine effectiveness was the cause of the outbreak. More effort is needed to increase vaccine coverage to at least 95% in all population subgroups.