Travel time to vaccine providers in health posts appeared to be a barrier to the delivery of infant vaccines in this remote community. However, there appeared to be no differential or modification of this effect by household wealth. We recently reported that geographic access to the major health centre in the study area was an important determinant of both early and late child mortality [9
]. Our analyses now indicate that travel time also effects the implementation of preventative interventions such as infant vaccination.
Our findings were clearly defined for all vaccines, though the effect was strongest for pentavalent vaccine. This may be due to the fact that pentavalent vaccines are only delivered at the static health posts while Measles vaccines are given through supplemental immunisation activities [16
] within the villages and BCG vaccines are given at the time of delivery.
This appears to be the first study that has examined associations between geographical access to health facilities and childhood vaccination coverage throughout infancy in such a remote area of Africa. This also appears to be the first study that reports that Measles vaccination may be less effected by geographic disparities than other vaccines. Studies from Tanzania [19
], Malawi [20
], Nigeria [21
], Kenya [22
] and Papua New Guinea [23
], all indicated that travel time to health facilities was a barrier to receipt of all infant vaccines including Measles [23
]. Studies from more densely populated areas such as Kenya found no association between the coverage of any infant vaccines and travel time to health facilities [24
]. This was thought to be due to the high density of health centres and health posts rather than the effect of supplementary immunisation activities or other Ministry of Health strategies.
Interestingly there was a marked association between vaccination coverage and kebele.
This may be due to differences in quality of services or other barriers such as social constraints or staffing levels in the different kebeles
. However, we did not adjust the results for travel time or other potential confounding factors and further analyses are needed to explore this issue. There was also little variation in vaccination coverage by household wealth, maternal age or education. This is likely to be due to the marked homogeneity in our study population and we reported similar effects in our earlier mortality study [9
There were several limitations to our study. Firstly, we were not able to assess coverage of important vaccines such as Polio due to problems with reporting. Secondly, we were only able to assess effects on vaccination coverage and could not examine effects on health outcomes such as vaccine preventable diseases or hospitalisations due to limited resources. Thirdly, as this was a cross sectional study, travel time was assigned at the point at which the mothers were interviewed and there was no ongoing tracking of migration. Thus there may have been some misclassification of travel time status. Fourthly, the timeliness of the vaccination was not evaluated in this study as mothers recall was considered to be too poor. Finally, we may have overestimated the percentage of vaccinated children as we only included surviving children in our study and we might have missed unvaccinated children who had died [27
]. In addition, our coverage data were calculated using children aged 12–59
months and are likely to be higher than other studies and the DHS which use younger children aged 12–23
]. Five to seventeen percents of vaccine data was also unknown or missing in our study, however, any differential misclassification of vaccination status would have tended to underestimate rather than overestimate effect sizes. Also, important strengths of our study were our careful collection of health facility data on vaccination and our reporting of potential confounders such as maternal education and household wealth. We also used a three-level-level random effects model to account adequately for clustering by mother, village and kebele. We also used a validated access measure [30
] (travel time) from each household to the health posts which accounted for the influence of topography and other natural barriers.
Our study has important implications for policy and program development. Vaccination coverage can be improved in remote areas by improving access and reducing travel time to health facilities. Supplementary immunisation activities may also be contributing to improvements in measles immunisation coverage in later infancy and reduction in inequalities. Where appropriate, outreach programs and supplemental immunisation activities should be considered for raising coverage in remote areas. In addition, new vaccine delivery strategies are needed for the hardest to reach children in Ethiopia and other parts of sub-Saharan Africa.