Background
Despite the improvement in global immunization coverage, about 16 % of the worlds’ children had not completed the 3-dose diphtheria-tetanus-pertussis vaccine (DTP-3) series by 2013 [
1]. The estimated 2013 global DTP-3 coverage among children aged <12 months, which is a key indicator of immunization program performance, was 75 % in the World Health Organization (WHO) African Region, and 84 % worldwide [
1]. However, thanks also to the Expanded Program on Immunization (EPI), children immunization coverage in the WHO African Region is improving, even if still sub-optimal in many areas [
2].
The EPI was introduced in Cameroon in 1976 [
3]. According to the 2011 Cameroonian Demographic Health Survey (DHS) [
4], the coverage of the nine EPI vaccines in children aged 12–23 months was 57 % at the national level, and 59.3 % in the West Region. In this region, wild poliovirus (WPV) type 1 was isolated from two acute flaccid paralysis (AFP) cases in October 2013 [
5]. In 2013, other four cases due to circulating vaccine-derived poliovirus type 2 (cVPDPV-2) were reported also in the Far North Region of Cameroon. According to the Global Polio Eradication Initiative (GPEI), a total of nine cases of WPV-1 have been reported in Cameroon during the years 2013–2014 [
6], representing the first polio-outbreak reported in the country since 2009.
Vaccine preventable diseases outbreaks are linked to inadequate levels of immunization coverage [
5]. Several factors are associated with poor vaccination coverage in resources-limited countries. A multilevel analysis involving 24 African countries [
7] showed that breaks in childhood vaccination are linked, at the contextual level, to high community illiteracy rates, high country fertility rates, and living in urban areas, while, at the individual level, they are linked to poorest households, uneducated parents, parents with no access to media and/or with low health seeking behaviours; the relative effect of the above factors may significantly vary according to the geographical area [
7].
In Africa, a more detailed and comprehensive information at district level is necessary in order to develop and implement appropriate strategies for improving immunization coverage. In Cameroon, there are no studies, conducted at the district level, assessing factors associated with incomplete immunization. To fill this gap, we planned a community-based cross-sectional survey in the district of Dschang (West Region, Cameroon), a rural setting sited only 30 km far from the area affected by the polio outbreak [
8]. The objectives of this study were to estimate the districts’ coverage of routine vaccination program, to identify risk factors for incomplete vaccination status among children 12–23 months old, and to assess the risk of WPV spread in the study population.
Discussion
We assessed the routine vaccination coverage and risk factors for incomplete vaccination among children aged 12–23 months in the Dschang district, West Region, Cameroon. The survey was performed during a polio outbreak occurred in the same region of the country [
5,
8]. Thus, OPV-3 coverage data reported from our study may represent an indicator for the risk assessment of WPV spread in the study area. Vaccination coverage was assessed by card only and by card plus parents’ recall. Immunization cards were available for 50.2 % of the children. As shown in Table
6, the vaccination coverage observed in this survey was higher than that reported for the whole Cameroon by WHO [
28], and by the 2011 Cameroonian DHS for the whole country and the West Region [
4].
Table 6
Vaccines coverage (by card plus parents’ recall) compared to latest data for Cameroon from WHO and Demographic Health Survey (DHS)
BCG | 99.8 | 82 | 87.1 | 95.9 |
DTwPHibHepB – 1 | 98.8 | 95 | 85.5 | 94.4 |
DTwPHibHepB – 2 | 98.2 | 89 | 78.3 | 87 |
DTwPHibHepB – 3 | 94.8 | 89 | 68.4 | 75.5 |
OPV – 0 | 97.6 | n.a | 71.7 | 84.4 |
OPV – 1 | 98.4 | n.a | 93.3 | 92.5 |
OPV – 2 | 97.8 | n.a | 85.5 | 87.5 |
OPV – 3 | 95 | 88 | 69.8 | 76.6 |
Measles | 91.2 | 83 | 70.6 | 79.8 |
Yellow Fever | 91.2 | 83 | 69.3 | 77.7 |
Not vaccinated | 0 | n.a. | 4.5 | 2.3 |
Partially vaccinated | 14.1 | n.a. | 49.8 | 40.7 |
Fully vaccinated | 85.9 | n.a. | 50.2 | 59.3 |
Fully vaccinated ≤12 months a | 73.4 | n.a. | 45.1 | n.a. |
According to card plus parents’ recall, 85.9 % of the children of our survey were fully vaccinated. Vaccination coverage reported by immunization surveys in rural African areas ranged from rates as low as 26 % in Burkina Faso [
18] to 88 % in Ghana [
29]. Moreover, in our survey, there was no unvaccinated child, confirming the low rate (2.3 %) reported by the 2011 Cameroonian DHS for the West Region [
4]. For comparison, unvaccinated children ranged from 6.5 % [
24] to 24 % [
20] in surveys conducted in Ethiopia. According to immunization card plus parents’ recall, we observed 14.1 % of partially vaccinated children (15.5 % by card only), a lower proportion than that reported by surveys conducted in Mozambique (28.2 %) [
22] and Ethiopia (20.3 %) [
24]. Although our findings are indicative of a rather high adherence to vaccination schedule in term of completeness, 26.6 % of children did not complete the recommended doses within one year of life; no comparable data were found in the scientific literature. DTwPHibHepB-3 coverage in our survey was 94.8 % by card plus parents’ recall (92.8 % by card only), which is higher than what reported by other surveys performed in rural Ethiopia (47.9 %) [
20], Nigeria (80.8 %) [
16], and Kenya (90 %) [
19]. OPV-3 immunization coverage was >90 %, based on both card plus parents’ recall and card only, indicating that 12–23 months old children living in the Dschang district had a good protection level against the risk of WPV spread related to the ongoing polio outbreak in the West Region of Cameroon [
5,
8]. Other surveys performed in Africa reported OPV-3 coverage of 91.8 % in Kenya [
19], 80.8 % in Nigeria [
16], 86.3 % and 54.3 % in Southern [
24] and Central Ethiopia [
20], respectively.
Limited differences were found in vaccination coverage when assessed by card plus parents’ recall or by card only. An expected tendency of highest vaccination coverage values when assessed by card plus parents’ recall was observed, in accordance with WHO and the international literature [
10,
20]. The biggest difference was related to OPV-0 (+4 %), though it was lower than that reported in a study from Ethiopia [
20], where differences ranged from 12 % to 34 %. In our study, as also reported by the 2011 Cameroonian DHS [
4] and by another survey from Kenya [
30], OPV-0 coverage (by card plus parents’ recall and by card only) was lower than BCG and OPV-1 coverage. The reason of this finding is undefined; OPV-0 could be underreported in the card by health care staff and/or less recalled by parents when compared to BCG vaccine, that is intradermally administered at birth, and to OPV-1 that is administered with DTwPHibHep-1 at 6th weeks of life. No difference was observed between measles and yellow fever vaccination coverage when assessed by card plus parents’ recall or by card only (Table
3), consistently with data reported by the 2011 Cameroonian DHS [
4] and by a study from Sierra Leone [
31]. This finding suggests that these two vaccines are correctly administered during the same vaccination session. The vaccination dropout rates observed are acceptable, according to the WHO standard (cut-off < 10 %) [
26], and lower than those reported by studies from Ethiopia [
23,
24].
Several characteristics were associated with immunization status in our study, but only six of them were statistically significant at the final multilevel regression model (Table
5). Birth order was statistically associated with fully immunization, similarly to what reported by studies from Kenya [
19] and India [
32], but in contrast with a survey from Ethiopia [
20]. This finding could be related to a reduced mothers’ attention along with growing number of children, due to an increase of duties, as confirmed by mothers’ interview (the main reported reason for not being able to vaccinate their child was “to be very busy”). Being born at the hospital/health facility was linked to complete vaccination status in studies from different African countries [
19,
20,
22], while in our survey it was associated in a restricted multilevel model including only the children characteristics, but the association was no longer statistically significant when adjusting for all other factors. Perhaps, the effect of place of birth was explained in the model by prenatal visits and/or access to health services, of whom it may be considered a proxy. A lower number of mothers’ ANCs and the younger mothers’ age were associated with incomplete vaccination status, as in other studies from African countries [
7,
16,
20,
29,
33]. It is possible that reduced contacts with health facilities during pregnancy, especially among younger mothers, could cause poorer information toward immunization. A longer distance (≥8 Km) from vaccination sites was marginally significant (
p = 0.05) in the final model. The accessibility to vaccination sites was associated to immunization status in studies from rural Mozambique [
21] and Nigeria [
16], while no distance related difference was found in a study from Burkina Faso [
18]. Mothers’ education was associated in the first multilevel model, including only parents’ characteristics, but no more when all factors were included in the model; this covariate was found associated with complete vaccination in several studies from African countries [
7,
16,
18,
29].
A high vaccination knowledge score, positive attitudes toward vaccination, good perception of immunization services, and the exposure to information on vaccinations, were all associated with complete vaccination status. As in other studies from Nigeria [
16], Niger [
33], and India [
32], despite a parents’ poor vaccination knowledge score in 50 % of the cases, the majority of interviewed parents (84.5 %) had a positive attitude toward immunization. Overall, only parents’ positive attitude and regular information received remained significantly associated with complete vaccination in the final multilevel model. These two factors were reported as determinants of complete immunization status in a multilevel analysis involving 24 sub-Saharan African countries [
7]. Concerning these aspects, it is noteworthy that the father of the index case of the polio outbreak recorded in the West Region in Cameroon (October 2013) was a political leader and pastor that, despite its good education level and high economic status, was strongly opposed to vaccination [
8], and his children had never been vaccinated.
Other demographic factors linked to vaccination status identified in studies conducted in several African countries were found either not statistically associated or associated only at the univariate analysis, or were just not considered in our study. Among these factors, parents’ socio-economic status and number of children in the family were not significant in the final multilevel model, while they were significantly associated in other African studies [
18,
28], including one from Cameroon [
34]; birth season and residence area, significantly associated in a study from Burkina Faso [
18], were not considered in our survey. Surprisingly, in contrast with the results of surveys conducted in African countries [
16,
18,
22] and India [
32], possessing the vaccination card was associated with incomplete vaccination. This, as suggested by WHO [
10], might be due to parents over-reporting of vaccine doses for complacency, without the possibility to check the information. This aspect is controversial and may represent a limit of our study. In fact, a recent systematic review on the validity of vaccination card and parents’ recall to estimate vaccination coverage [
35] suggests that parents’ recall information should be cautiously interpreted because it might be not reliable. This issue is not mentioned in WHO EPI-coverage survey guidelines [
10] and has not been addressed in the fields’ immunization surveys [
16‐
24]; thus, methodological studies on this aspect are needed.
Another limit is that our survey was conducted in a district of a region affected by a polio-outbreak situation, and two SIAs have been realized before and during our study. This situation could introduce biases in the OPV coverage evaluation for different reasons: firstly, vaccine doses may have been administered from other than Dschang vaccination facilities; secondly, parents could incorrectly report vaccine doses as administered during SIAs or routine immunization and vice-versa. Thus, the contribution of routine immunization activities and SIAs to the OPV coverage rate could not be completely distinguished. However, coverage rates for OPV-3 were similar to DTwPHibHepB-3, as well as all observed coverage for the different vaccines administered during the same routine immunization session. Furthermore, even if the parents’ tendency to over-report doses of vaccines [
10,
20] may introduce biases, vaccines coverage rates assessed by card plus parents’ recall and by card only were similar in our study.
A further study limitation concerns the procedure used to determine households’ economic status. In fact, our specific algorithm was based only on the type of family (two or single-parent) and parents’ occupation, while housing conditions and economic activities of other household members were not considered. The sampling procedure was also susceptible of selection bias and the study did not include qualitative methods to address this questions. Moreover, qualitative methods, such as in-depth interviews, were not used to investigate the reasons for partial or delayed vaccination. Planning qualitative studies may be a useful complement to quantitative surveys in better understanding obstacles and possible solutions to increase vaccination coverage.
To our knowledge, despite the above mentioned limitations, this is one of the few studies using a multilevel logistic regression analysis to identify risk factors for incomplete immunization status and the intracluster correlation coefficient for the sampling size calculation. Our study contributes to the identification of factors related to the children immunization status at a district level in Cameroon, where similar studies are lacking. Moreover, we reported the vaccination coverage in children <12 months of age, that is not mentioned in surveys performed in similar African contexts, and we provided an important risk indicator of WPV spread during the ongoing outbreak situation.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
GianlucaR conceived of the study, participated in its design and coordination, data collection and interpretation, and drafted the manuscript. AM conceived of the study, participated in its design, data collection and interpretation, and drafted the manuscript. PP conceived of the study, performed data analysis, and participated to data interpretation. RMB participated in the design of the study and coordinated data collection. MSS conceived of the study and participated in its design and data collection. GBM participated in the design of the study, data collection and interpretation. PS: conceived of the study and participated to data interpretation. VV conceived of the study, participated in its design and data interpretation. GiovanniR conceived of the study and participated in its design, data interpretation and manuscript revision. All authors read and approved the different study steps and the final manuscript.