Assessing participation bias in a population-based study of measles-mumps-rubella vaccine immunity in children and adolescents aged 12-18.

The purpose of our study was to determine whether specific characteristics were associated with study participation in a group of children residing in Olmsted County, MN. We compared 346 participants and 848 non-participants from a study examining associations between human leukocyte antigen gene variants and immunity following measles-mumps-rubella (MMR) vaccination by demographic characteristics, MMR vaccination history and length of time in the Olmsted County health care system. We also compared the frequency and reasons for health care visits between participants and non-participants by comparing diagnostic codes for all visits that had occurred between 1999 and 2001. Characteristics were compared using chi-square and t-tests, followed by multivariable logistic regression. Study participants were more likely to be white/Caucasian, to have received their first MMR vaccination at a younger age, and to have had more health care visits (especially for acute respiratory illnesses, vaccinations, or other acute conditions such as fainting and headaches) than study non-participants. These results suggest that frequent use of local health care systems by children and parents may increase comfort levels with local physicians and physician researchers, thereby improving participation rates in research studies among these populations. However, special efforts may be necessary to improve research participation among children who are infrequent users of the health care systems of interest.