As expected, on the basis of high vaccination coverage for infants for the vaccine-preventable diseases in the NIP [1
], the attitude towards vaccination was generally positive. Not surprisingly, we observed that members of Reformed Congregations, Reformed Bonders and those with anthroposophic beliefs participated less in the NIP than those with no religion or no anthroposohic beliefs. We also observed other groups who participated less in the NIP compared to the reference categories, including those of non-Western descent, with a low income and with a low educational level.
Of interest, the analyses of population-attributable fractions showed that with regard to nonparticipation in the NIP, particular attention ought to be given to individuals with Moroccan or Turkish ethnicity and those with low educational and income level. They seem to contribute in large measure to the rate of nonparticipation in the NIP, i.e., to a greater extent than well-known vaccine refusers such as specific religious groups and anthroposophics.
Lower NIP participation among those with a low educational level or income compared to those with a high educational level or income is consistent with the recent observation that in areas with low socioeconomic status (SES) compared to high SES the uptake of human papilloma virus (HPV) vaccination in the catch-up campaign in 2009 was relatively low [16
]. Furthermore, in a systematic review by Brown et al. [17
], a lower vaccination uptake was linked to lower parental education and income compared to higher parental education and income. In contrast, several other studies showed a more negative attitude/intention or higher prevalence of nonadherents in relation to future vaccinations among persons with a higher educational level compared to those with a lower educational level [18
]. Similar to our finding that NIP participation was lower among non-Western migrants compared to indigenous Dutch, the uptake of HPV vaccination was found to be lower among non-Western ethnicities than among indigenous Dutch, in particular, individuals of Moroccan and Turkish descent [16
]. Other studies have observed a higher vaccination uptake among persons of Moroccan and/or Turkish descent compared to indigenous Dutch [21
]. In the systematic review by Brown et al. [17
] both a higher and a lower vaccine uptake was observed for non-white ethnicity compared to white ethnicity. We hypothesized that the lower observed NIP participation might be caused by a potential language barrier, as the invitation letter and information on vaccines included in the NIP they receive at home are in Dutch. They have to visit the NIP website to read the information in their own language. An evaluation might be undertaken to determine whether offering non-Western migrants more written and oral information in their own language would result in a higher NIP participation in this group compared to the daily practice as was described above.
No associations for religion, income and educational level were found with parents' intentions to accept any remaining vaccinations for their child in contrast to our findings for NIP participation. However, a similar trend was observed. It might be that numbers were too low to observe an association as only a subset of the population was used. Univariately, anthroposophic/homeopathic or alternative medicine beliefs were associated with parents' intention to accept any remaining vaccinations for their child. After adding statements regarding vaccination concerning safety, maximum number of injections, protection of the health of one's own child and whether vaccinating healthy children is necessary, the effect was not significant anymore. These parental concerns have been identified previously in the literature as associated with vaccination behaviour [10
]. The vaccination statement "no good development of immunity" has been found in literature [10
] to be associated with vaccination behaviour. Univariately, a high odds ratio was observed with those less likely to accept any remaining vaccinations, but by adding the other vaccination statements in the multivariate model the effect was no longer significant. Among those with low SES or of Moroccan or Turkish descent, it was not possible to study which opinions on vaccination were associated with parents' intention to accept any remaining vaccinations for their child. More (in-depth) research would be needed to verify why they are participating less in the NIP and to better understand their attitude and beliefs regarding the NIP.
Some limitations of this study must be discussed. This research was part of a larger seroepidemiological study and therefore, the number of questions related to attitude towards vaccination was limited. We also could not rule out any influence by reporting bias. A short pilot for the questionnaire was performed. The calculation of a PAF presumes that the effect of the determinant that is changed on vaccination rates is a causal effect. Such causality can not be inferred from our cross-sectional study. Therefore the PAF should be interpreted as the magnitude of the improvement of rates of reporting participation in the NIP that is maximally possible by removing the exposure to nonparticipation in the NIP if the effect of this exposure would be causal. Furthermore, overall responses of those who filled in a questionnaire of 66% and 40% in the first and second survey respectively, made nonresponse bias possible. The availability of background information made it possible to weight the PAFs for the variables known to be related with nonresponse, e.g., age, gender, ethnicity, region and degree of urbanization [28
]. However, nonresponse bias related to other factors could be present.
It might be worrisome that both in the NS and in the LVC sample, participants in 2006-07 reported to be less inclined to accept vaccination than in 1995-96. To maintain trust in the vaccination program, we need to monitor the acceptance of the NIP in a timely fashion and try to find reasons behind a more reluctant attitude. It is likely that doubts regarding the decision to have children vaccinated precede changes in behaviour; use of our monitoring system for vaccination coverage will therefore probably be less sensitive and too slow. Future research will therefore focus on developing a system to monitor the acceptance of vaccinations among parents and (child) health care professionals and to review blogs and forums about vaccination on the internet. This system could be used to develop novel and targeted interventions to increase vaccination acceptance.