This study describes immunization coverage and risk factors for incomplete vaccination for children aged 12-23 months in urban informal settlements in Nairobi. The study shows poor full immunization coverage according to WHO recommendations for full vaccination at 58%. Immunization coverage was particularly low for measles at 62%. The study also indicates even poorer up-to-date immunization coverage at only 52%. Significant determinants of complete vaccination in these communities included poverty, ethnicity, place of delivery, mother's education and parity of the mother. These results clearly indicate important areas for intervention to improve immunization coverage among the urban poor in sub-Saharan Africa.
The full immunization coverage for children aged 12-23 months reported from vaccination cards in this study is low compared to that reported nationally, in urban areas and even in rural areas in Kenya. According to the 2008-2009 Kenya Demographic and Health Survey, the national coverage for full immunization is 77%, while that of Nairobi as a whole is 73%. Urban areas in Kenya have the highest level at 81% while that of rural areas is 76% [4
]. The results in this study indicate that children living in urban informal settlements are the most disadvantaged sub-group and do not benefit from the urban advantage. While there is an improvement from results of a study carried out in the Nairobi slums in 2000 which showed that full immunization coverage was 44%, the improvement is small compared to that from 57% to 77% reported nationally for Kenya between 2003 and 2008 [4
]. The full immunization coverage compares with that of other slum areas in developing countries [13
]. For example, while full immunization coverage among children aged 12-23 months in Bangladesh as a whole is 75%, only 54% of children of the same age are fully immunized in the urban slums of Dhaka [14
Various reasons may explain the lower levels of full immunization coverage in urban slums in the developing world. A comprehensive review of immunization services in urban areas in developing countries identified several challenges that are unique to urban areas [15
]. These include rapid population growth particularly in slum populations, array of types of service providers in both private and public sectors, other more pressing challenges that need to be prioritized and the need to use creative strategies to reach marginal sub-populations [15
]. Similar challenges face the urban poor populations in Nairobi. In addition, being illegal settlements, slum areas in Nairobi have in the past had little recognition from the government and therefore have been marginalized with regards to provision of basic services. Although immunization services are offered free of charge at public health facilities, these facilities are often not found in the slums and slum dwellers have to seek for public services outside of the slum areas. They otherwise seek services offered by private practitioners in the slums who are mostly illegal and offer sub-standard and relatively expensive services [16
]. This limits access to basic services such as vaccination services and may be the main reason for low full immunization coverage.
While achieving high immunization coverage against vaccine-preventable diseases is of significant public health importance, achieving a high level of UTD vaccination coverage may even have higher-reaching benefits. Using vaccination card data, this study indicates a lower level of UTD vaccination than that of full coverage, declining from 66% for vaccinations that should be completed by 3 months including BCG, polio 1, 2, and DTP/pentavalent 1, 2, to 52% at 12 months when all vaccines should have been given. Evidence indicates that giving vaccines at the wrong time may have adverse implications [17
]. For example, studies in West African countries indicate that DTP given concurrently with or after measles vaccine is associated with higher mortality than having measles vaccine alone as the latest vaccine [18
]. Further, a study in Bangladesh showed that giving BCG and DTP after the age of measles vaccine increased mortality by three-fold compared to not vaccinating at all [19
]. This indicates the importance of emphasizing immunization on schedule for children living in informal settlements.
The study indicates high vaccination coverage for vaccines that are given within the first few months after birth at between 87% and 98%. On the other hand, coverage for measles, given towards the end of the first year was poor with only 62% receiving the vaccine. This difference in coverage between these two sets of vaccines has also been documented in other studies in developing countries, [13
] and may be due to the long interval between them. The low immunization coverage for measles is of particular public health concern given both the specific and non-specific beneficial effects of measles vaccine on childhood morbidity and mortality documented in several studies in the developing world [22
]. The other concern with the low coverage of measles vaccination is the absence of herd immunity. High measles coverage provides herd immunity thereby decreasing the risk for measles exposure and affording protection to the small proportion of individuals who are not vaccinated.
This study also reveals the extent of missed opportunities for vaccination in the slum settlements. For example, while BCG vaccination coverage is almost 100%, close to 20% of children do not receive the zero dose of polio given at birth together with BCG. Although the birth dose of polio is not within the WHO guidelines for full immunization, it is important for the efforts towards polio eradication. In addition, while only 3% are not given DTP3/pentavalent3, 13% are not given OPV 3 dose given at the same time. A few explanations may be given for the disparities: evidence in other slums indicates lack of confidence of health workers in administering two vaccines at the same time owing to fear of contraindication. Furthermore, shortage of vaccines and the unwillingness of health workers to open vaccine vials if there are not enough children needing the vaccine at a given time have been cited as reasons for these differences [24
]. The results indicate significant delays in BCG and OPV-0 vaccinations immediately after birth. The delay of 11 days observed suggests that supply of the vaccines at health facilities which conduct deliveries may not always be assured at the time of birth. The delay may also be associated with mothers who deliver outside of formal health facilities but are aware of the need for these vaccines and take the children for vaccination at health facilities a few days later. The substantial delay observed when the two vaccines are administered at different dates or when only BCG was given suggests that vaccine shortage at the health facilities at time of delivery problems is a major factor in the delay or non-receipt of BCG or OPV-0 vaccinations. These challenges with vaccine stockouts are common among the numerous privately run health facilities in Korogocho and Viwandani.
From the experience working in the community, mothers often report shortage of the polio vaccine. This irregularity is not often reported with regards to BCG and DTP/Pentavalent. With regards to OPV zero, there is a duration limit of 14 days within which the vaccine should be given, if children report after these 14 days, they are given OPV 1 instead. The higher coverage for DTP3/pentavalent compared to OPV3 may be due to donor emphasis on using DTP3/pentavalent to monitor and evaluate performance of vaccination programs. However, there is need for further investigations as there seems to be no disparity between OPV 1 and 2 and the respective DTP/pentavalent doses. These findings indicate a need for increased awareness amongst health care workers to ensure administration of all eligible vaccines at the time the caregiver visits the clinic for vaccination of a child. Further, the government may need to ensure regular supply of polio vaccine.
The finding of greater likelihood of full vaccination among children born at a health facility has been reported in previous studies [26
]. Mothers who deliver at health facilities may be more frequent users of health facilities and services including immunization for children. The administration of BCG and OPV at birth is required at registered health facilities and may partly account for the higher proportion of facility-delivered children with full vaccination. Home deliveries in urban slums and rural areas are often conducted by traditional birth attendants and usually do not involve administration of vaccines [27
]. Other studies have shown that maternal education, attendance for antenatal and postnatal care, and parity are associated with full vaccination among children [8
]. In this study, women who received antenatal and postnatal care were not significantly more likely to have a fully vaccinated child compared to those who did not. A high proportion of women in the two slums reported utilizing antenatal and postnatal services. However, the quality of antenatal and postnatal care services provided at health facilities serving the slum population is generally substandard and may not include components such as immunization which these services are expected to provide [30
]. The study also showed that mothers resident in the slums who had higher parity were less likely to ensure that their children received all the required vaccinations. This relationship has been shown in other studies in non-slum areas of Kenya and has been linked to the higher cost and demands on resources caused by having more children, which may adversely affect healthcare utilization [29
]. The greater vaccination coverage among children resident in Viwandani suggests the presence of contextual factors within the slum that influence the decision of caregivers to get their children vaccinated. Such contextual factors may include level of education in the slums (maternal, paternal and other household members). The higher vaccination coverage in Viwandani may be because this area attracts migrant workers with relatively higher education levels, which may influence vaccination coverage at the community level. Additionally, given that public health facilities where slum residents may seek vaccination services are located outside the slums, Korogocho residents, being of lower socio-economic status may be limited by transport costs. Within both slums, greater likelihood of full childhood vaccination was associated with being of the Kikuyu ethnic group. This is consistent with previous reports for the whole of Kenya in which the Kikuyu children or children from Central Province where this ethnic group mainly lives had the highest odds of being fully vaccinated or being at advantage with regards to other health outcomes [4
]. This suggests positive cultural characteristics of this group in relation to vaccination and health seeking behavior in general. It may also relate to higher socio-economic status or education attainment among Kikuyus (or people from Central Province) documented nationally [4
]. Anecdotal evidence from the NUHDSS data also suggests that Kikuyus have higher social economic status in the slums. However, further research may need to unravel the ethnic disparities with regards to vaccination.
The poverty variables used in the study included two measures of poverty, namely household assets and monthly expenditure. These variables reflect different dimensions of poverty in the slums. Household assets is a slow-changing measure of poverty and was associated with full vaccination. Children in villages with greater household assets and expenditure were more likely to be fully vaccinated than those in villages with less household assets. Assets are a reliable measure of the underlying structural well-being of a household whereas household expenditure demonstrates much greater periodic fluctuation [33
]. This distinction may explain the extent of association of these measures with full vaccination of children in the slum. There were similarities in the odds ratios of the variables in the two models using each measure of poverty, suggesting that household assets and monthly expenditure are correlated in the slums and do not individually alter the association of the various factors with the likelihood of vaccination. Financial barriers among the socio-economically disadvantaged groups have been shown to predict under-vaccination although these are typically represented as costs or relative measures such as wealth quintiles [34
]. This study, which explores specific dimensions of poverty in the resource-deprived slums of Nairobi, demonstrates that household economic deprivation when expressed at community level influences the likelihood of full childhood vaccination.
This study has a number of strengths and limitations. Vaccination status was determined using data from vaccination cards thereby ensuring the accuracy of information. This eliminates the recall bias that exists in cases where vaccination cards are unavailable and researchers have to rely on the mother's report. Only a small proportion of children (16%) did not have vaccination cards, compared to 30% nationally and 58% for Nairobi Province reported in the 2008-2009 Kenya Demographic and Health Survey [4
]; these were omitted from the regression analysis. The longitudinal nature of the study ensured timely updating of the vaccination data, hence reducing the bias that would have otherwise been created by loss of vaccination records. The inclusion of children born throughout the duration of the MCH study ensured a larger sample size but assumes that the influence of maternal factors on vaccination status is the same throughout the study period.