An outbreak of measles in a rural Queensland town in 1997; an opportunity to assess vaccine effectiveness

This report describes a measles outbreak in a rural town in south-east Queensland and presents the results of a vaccine effectiveness (VE) study performed during this outbreak. It is important to assess the effectiveness of a vaccine in an outbreak to determine if the outbreak is due to failure of the vaccine or failure to vaccinate. There were 44 cases of measles amongst local residents, which represents a notification rate of 396.7 per 100,000 population. Case investigations identified a group of people who had been exposed to measles at a seminar. The attack rate for the seminar cohort was 18% (11/61). This presented an opportunity to conduct a VE study using data about children aged less than 16 years who attended the seminar. In this cohort of 23 attendees, all 7 children who had not received any measles vaccinations became cases whilst the 6 who were fully vaccinated for their age according to NHMRC guidelines were protected from measles illness. Although there were insufficient fully vaccinated cohort members to reliably estimate VE for this group, the vaccine was 84.6% (95% CI: 15.0-99.7%) effective for those who had received at least one validated dose of vaccine. Despite the sample size limitations, the results support the view that failure to vaccinate rather than vaccine failure contributed to the high infection rate in the seminar cohort. Commun Dis Intell 1999;23:240-245. Investigation of the education seminar cohort identified an opportunity to assess the effectiveness of the measles vaccine. Assessing the effectiveness of a vaccine in an outbreak is important to determine if the outbreak is due to failure of the vaccine or failure to vaccinate. Vaccine efficacy is usually determined under controlled conditions prior to licensing, however the effectiveness of a vaccine under normal (field) conditions may vary and should be assessed when the opportunity arises. Vaccines may fail due to incorrect storage and administration of the vaccine, drug interference or the age at vaccination. Vaccine