This study assessed the complete immunization coverage and factors associated with it among children aged between 12–23 months old in Ambo Woreda of Oromia Regional State found in Ethiopia. Based on immunization card and recall, 35.4% children were fully vaccinated, and 23.7% were unvaccinated. The pentavalent3 coverage was 47.9% and 54.9% took measles. The OPV vaccine coverage was slightly higher than the coverage of the pentavalent vaccine. The measles coverage was higher than the pentavalent3 coverage. But it is expected that the pentavalent3 coverage should be higher because of dropout and the long time gap between the two vaccines, in which the mother may not return back the measles vaccine. The higher coverage of measles and OPV vaccination was assumed to be due to the frequent national campaign that focused on the two vaccines.
Compared the immunization coverage of Ambo Woreda with the EDHS 2005, the proportion of children fully vaccinated in the present study was higher by 15%, but it was similar by proportions of unvaccinated children . Beside this, the current findings is higher than the 2006 immunization coverage survey in the country which estimated fully vaccinated by card is only at 20% . But it is lower than the immunization coverage reported in the 2008 health and health related indicators  and 2010 woreda health office report. This difference is may be due to the over reporting of health and health related indicators data from some areas.
From the total interviewed households, 224 (41.8%) mothers showed the vaccination card of their children. From the card most children took BCG and OPV1 vaccines, but only 27.8% of finished the immunization. The coverage by card only was also less than that of health and health related indicators. The proportion of fully vaccinated children in this study was about 8% higher than the EDHS 2005 and 2006 EPI survey coverage [10, 18, 19]. This difference is because of the result from such country level study includes area of low immunization coverage.
Apart from this, in this study mothers’ knowledge on vaccination and vaccine preventable disease was also assessed. About 96% mothers heard about child immunization and vaccine preventable disease, but only 79.5% of them mentioned that vaccination is used to prevent disease. About 20% of respondents knew four and above disease which are preventable vaccines. Regarding knowledge of mothers about the age at which the child begins and finishes the immunization, less than half of the respondents knew the correct age at the child begin immunization; and 67.5% of them knew the correct age at which the child should finish vaccination. In addition, only a fourth knew the number of sessions needed to complete immunization. The knowledge of mother is significantly different for respondents from urban and rural areas. This finding is consistence with the study done in Nigeria in which, more than half of mothers knew the purpose of immunization, but it is not similar with this study for the knowledge of the schedule of immunization . The explanation may be related to the educational status of the mothers in which most of them are illiterate this study.
In this study age of mother, educational status, marital status, occupation and place of residence of the mothers did not show significant association with the completion of immunization among children aged between 12–23 months. This finding is similar with the case control study done in Wonago Woreda, Southern Ethiopia . However, previous studies done in Sudan and other part of Ethiopia indicate that these factors have a significant association with completion of child immunization [11, 21–23].
However, in this study children delivered at health facilities were more likely to be fully vaccinated (OR = 2.6) than children delivered at home. This finding was similar with the study done in Mozambique in which children delivered at home were less likely to complete immunization . The explanation related to this may be that, mothers who gave birth at health institution are closer to the health service and most of the time the first dose of vaccination is given just after birth in health institution.
Besides its relation with institution delivery, complete immunization coverage of children showed statistically significant association with mothers’ utilization of antenatal care (ANC) follow up. Children of mother who had ANC were 2.1 times more likely to complete vaccination than those with no follow up. This is consistent with the study done in India and Bangladesh in which ANC follow up is related with complete immunization coverage [14, 15].
In Ethiopia it was indicated that, lack of awareness about immunization contribute to low immunization coverage in Ethiopia . The findings of this study also showed that lack of knowledge about vaccination and vaccine preventable disease, and age at the child begins and finishes the immunization, and is related with completion of immunization among children aged 12–23 months. Children of mothers those knew the correct age at which the child begins and finishes immunization are more likely to complete immunization than those who did not knew. These finding is consistent with the study findings from Tigray and Wonago in Ethiopia [12, 18]. However, knowledge of objective of vaccination had no significant association with complete immunization. This indicates that knowledge of mothers about the schedule of immunization is more important in completion of the recommended immunization for the child than knowledge of the purpose of vaccination.
This study assessed full immunization coverage among children aged 12–23 months with regard to the recent immunization program performance. However it has certain limitations.
Report by the mother may under/overestimate the immunization coverage. Because mother may forgot the total doses of vaccine that the child took. Beside, this study did not consider the validity of the doses of vaccines child took. Also, sampling procedure was susceptible to selection bias and the study did not include qualitative method to answer the why questions. Moreover, problems from the health facility perspectives were not addressed by this study. Despite the above limitations, our findings are important to understand factors associated with immunization completion among children.