Refusal to Participate in Blood Testing in a Study of Childhood Immunizations and Atopic Disorders: Characteristics of Non-Participants and Assessment of Possible Bias

Aim: Assessing characteristics of non-participation in epidemiological studies is often complicated by lacking information. The aim was to assess characteristics of nonparticipants in blood testing and possible nonparticipation bias in our previous study on the impact of vaccinations on atopic disorders. Methods: In a previously conducted study on vaccinations and allergy we now used multivariable logistic regression to assess characteristics of non-participants in blood testing, an optional part of the study protocol. Possible bias due to this non-participation was assessed by an analysis weighted with the inverse of the probability of being a participant and by a sensitivity analysis. Results: Having refused consent to consult vaccination registration data (OR: 4.7, CI95%: 2.9-7.6), not having disclosed income, lower school class, lower birth order, not having a history of pertussis, and eating less vegetables were significant determinants of non-participation in blood testing. Weighted analysis and sensitivity analysis yielded results similar to those in the original study. Conclusions: We found that refusal to participate in blood testing was related to reluctance to disclose private information in general and to sensitivity on the subject of vaccinations in particular. Also, parents of younger children with less older siblings, without a history of pertussis, and consuming less frequently vegetables, were more likely to be a non-participant. Selective participation in blood testing may have affected our assessment of the reliability of the reported vaccination status, but leaves our conclusion from the original study, that there is no positive association between the DTP-IPV vaccination and atopy, unaffected.