BACKGROUND:
Experience from Oregon suggests two separate sources of religious/philosophical exemption seeking. The first is from ‘free-riders’ who take advantage of herd-immunity built by others, but who take convenience exemptions for their own children. The second source is from anti-vaccine sentiment. Free-riders are broadly diffused, and may not pose a significant public health concern. In contrast anti-vaccine sentiment is likely to be concentrated in local communities or social networks, and strongly impact local herd immunity. Rates of religious exemptions in larger areas may not indicate whether pockets of anti-vaccine sentiment exist locally.

OBJECTIVE:
To measure both local and aggregate rates of exemptions, for the purpose of determining background rates and identifying communities with pockets of exemptions.

METHOD:
Exemption rates were collected for Oregon public schools for the 2002-2003 school year. Individual school rates were aggregated and mapped by community, school district and county.

RESULT:
Among 1,181 public schools, the mean rate of exemption was 1.96%. However of 9,018 reported exemptions, 55.4% occurred among 321 schools with a rate above the 99% confidence level, (>2.18%), and 24.5% occurred among 90 schools with a rate greater than 5%.

CONCLUSION:
Exemption rates in Oregon appear to be a combination of a broadly diffused background rate plus local pockets of higher rates. This finding is consistent with the view that exemption-seeking draws both from free-riders and from pockets of anti-vaccine sentiment. These pockets cannot be predicted by looking at rates for larger areas, such as counties. This finding has important implications for understanding local herd immunity levels, and for designing exemption surveillance and surveys.

LEARNING OBJECTIVES:
Participants will learn about these two types of exemption seeking, and how they differ both in location and in implications for public health.