Assessment of School Immunization Coverage Using Random Sampling and CoCASA

Background:
New Mexico has traditionally calculated school immunization rates from rosters submitted to the Department of Health (DOH) by schools themselves for kindergarten, first, and seventh grades. Starting in the 2007-08 school year, the DOH opted to conduct surveys of randomly-chosen kindergarten and seventh grade classes across the state. Schools were informed they would no longer need to submit annual immunization rosters to the DOH. Instead, all elementary and middle schools had a chance of being selected at random as part of the new school survey.

Objectives:
Survey objectives were to measure compliance with New Mexico school entry immunization requirements and ACIP-recommended schedules, and to measure rates of religious, medical, and conscientious exemptions. Budget limitations dictated that the survey be performed as efficiently as possible.

Methods:
Twenty kindergartens and 20 middle schools were selected using probability proportional to size sampling. Up to 30 students were chosen at random from each class. School nurses or administrators accessed individual school records and provided immunization history information to reviewers who then entered it into CoCASA. Files for individual kindergartens were merged for analysis in CoCASA, as were files for seventh grades. Rates were calculated for individual antigens and applicable vaccine series.

Results:
Eighty-six percent (+/-3%) of kindergarteners and 72% (+/-3%) of seventh-graders received all immunizations required for New Mexico school entry as of September 1, 2007. Valid exemptions and ‘catch-up' immunizations brought school immunization compliance rates to 95% and 87% respectively by survey dates in early 2008.

Conclusions:
This sampling methodology using CoCASA as a survey tool was an efficient and statistically valid method for estimating school immunization coverage levels and compliance with school entry laws.