Measles outbreak in western Sydney. Vaccine failure or failure to vaccinate?

Abstract

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.