Local discrepancies in measles vaccination opportunities: results of population-based surveys in Sub-Saharan Africa

Surveys were conducted on average 30 days after the first day of the mass vaccination campaign (range: 1 day to 4 months). The age group included in the surveys varied according to the age group targeted by the ORI. All surveys employed probability proportional to size cluster-based sampling (Additional file 1: Table S1), except the surveys conducted in N’Djamena (Chad) in 2005 and 2010 which used Lot Quality Assurance Sampling. The first household to be surveyed in each cluster or lot was selected randomly by spatial-based sampling [8], GPS-based sampling, complete household enumeration or by the EPI method [10]. Method for selection of the first household was by preference by complete enumeration when logistical and security constraints allowed. Subsequent households were selected by proximity. Sample size varied according to the design and hypothesis about existing vaccination coverage (range: 942 to 6622 children).

Trained surveyors conducted face-to-face interviews with child’s main caregiver. Standardized, pre-piloted questionnaires were used and interviews were conducted in local language(s). Information was collected on demographic characteristics (age at the time of the survey or date of birth, sex) and vaccination status for different vaccine opportunities (Additional file 1: Table S1). When possible, information on vaccination history was verified with vaccination card. When card was not available, probing questions, such as place were the vaccination took place or injection site, were asked to minimize the risk of misclassification.

Vaccination status was collected for different opportunities. ORI implemented by MoH with the support of MSF (MSF/MoH ORI) and EPI were considered in all study sites. Although different SIAs had been implemented at different time points in these countries, only vaccination during the most recent was considered. Consequently, we considered SIA implemented in 2002 for the survey conducted in Mbuji-Mayi (DRC) in 2006; SIA implemented in 2008 for the surveys conducted in Malawi in 2010; and, SIA conducted in 2007 for the surveys conducted in DRC in 2011. In Maroua (Cameroon) and Kirundu (Burundi), SIAs were implemented before the ORI but were not included in the surveys. Lastly, in four countries (Cameroon, Malawi, Chad and CAR), we also evaluated MoH mass campaigns in response to outbreaks conducted before MSF/MoH ORI.

Administrative coverage was provided by local health authorities.

Children were considered as vaccinated through a specific opportunity if they were eligible for vaccination (i.e. in the target age group) and vaccination was reported by card examination or oral history.

Vaccine coverage and 95% confidence interval (95% CI) were estimated for each vaccine opportunity. EPI coverage was defined as the proportion of children vaccinated through the routine program among children aged 9 to 11 months (i.e. in the targeted window for this program) and 12 to 23 months (i. e. who should have recently received their MCV through EPI). SIA, ORI and other potential MoH reactive campaigns coverage were defined as the proportion of children vaccinated amongst the target age group at the time of the campaign. ORI coverage was calculated by age group.

If the delay between the last day of the MoH/MSF campaign and the first day of the vaccine coverage survey was <31 days, age at the time of the survey was considered to be similar to age at the time of the MoH/MSF campaign. If not, age at the first day of the MoH/MSF campaign was calculated.

To evaluate if children received the two recommended doses, we calculated the number of doses received by summing the number of occasions a child was considered vaccinated. Only children with all information on vaccine opportunities were included in this sub-analysis, otherwise number of doses was considered as missing. In Malawi and DRC, where EPI, SIA and ORI opportunities were evaluated, we estimated the proportion of children vaccinated through ORI according to the number of doses of MCV they received before, and the proportion of children for whom the ORI dose represent a first, a second or a third dose of MCV. For this sub-analysis, information on EPI vaccination was considered for all children aged 9 month or older included in the survey and not only for children 9 to 23 months of age.

To explore potential associations between vaccination coverage and explanatory variables, Poisson regression was conducted when several places were surveyed at the same time. Wald tests were used to test explanatory variables (p < 0.05). Estimations were weighed to account for survey designs. Data was entered in EpiData v3 (Odense, Denmark) and analysed with Stata v11 (College Station, Texas, USA).

Surveys were conducted as part of the monitoring and evaluation of the emergency response and were not submitted for formal ethical approval in the country of the study. The MSF Ethical Review Board exempts such surveys from review as they constitute part of the emergency response and use a standard and widely accepted survey protocol. All studies, however, were approved by local and national authorities through the convocation of exceptional meetings. In N’Djamena, surveys were conducted with the authorization of the national technical committee for the battle against epidemics; in Matadi and Mbuji Mayi (DRC), authorization was obtained from the Ministry of Health; in Maroua (Cameroon), a request was made to the Division of Operational Research within the Ministry of Health for specific ethical clearance for the survey and was granted; in Malawi, the study was implemented in collaboration with the MoH after obtaining permission to carry out the survey, in Katanga, authorization was delivered by national and provincial health authorities.

The main caregivers of all participants provided oral informed consent via a specific explanation in the local language as well as a written document providing information on the use of information and on confidentially. No specific ethnic or identifying information was recorded and all participants were free to refuse participation in the surveys. Vaccination and care were provided free of charge irrespective of participation in surveys.

Overall, MoH/MSF campaign coverage was >90% in most surveys conducted in DRC (except Malemba-Nkulu, Kambove and Kapolowe), Malawi (except Mzimba) and Burundi (Table 1). Coverage was >95% in 3 districts of DRC, including two urban districts (Likasi and Lubumbashi) and one rural district (Kipushi). Coverage was also >95% in two urban-rural districts of Malawi (Chiradzulu and Thiolo). The lowest MoH/MSF campaign coverage was estimated in N’Djaména (Chad) in the survey conducted in 2005 where less than 75% of children were vaccinated.

Estimated coverage varied within locations in Malawi (p = 0.004) and in DRC in 2011 (p < 0.001), but not in Moïssala district (Chad) (p = 0.07). In Malawi, the percentage of children vaccinated was highest in the rural area of Chiradzulu and lowest in the urban-rural district of Mzimba. In Katanga province (DRC), ORI 2011 campaign coverage varied from 99.3% in the urban zone of Likasi city to 81.1% in the rural northern district of Malemba-Nkulu. ORI coverage was significantly lower among children <1 year-old in the surveys conducted in N’Djaména (Chad) in 2010, in Maroua (Cameroon) and in Thyolo district (Malawi). ORI coverage did not vary by sex.

The surveys conducted in N’Djaména (Chad) in 2005 showed the lowest estimate with only around one third of children 12 to 23 months of age receiving MCV through EPI (Table 2). Burundi had the highest estimates with vaccination coverage above 90%. EPI coverage was close to 90% in all districts of Malawi. EPI coverage was significantly lower among children aged 9 to 11 than amongst 12-23 months old in Katanga province (DRC) (p = 0.01), Malawi (p < 0.001) and Moissala district (Chad) (p < 0.001).

Surveys conducted in diverse locations of the same country showed significant differences in Malawi (p < 0.001), and in DRC (p = 0.005), but not in Moïssala district (Chad) (p = 0.76). In N’Djaména (Chad) there was a significant increase in vaccination coverage from 2005 to 2010.

Administrative coverage overestimated survey results in Chad (N’Djaména 2005 and Moïssala 2011), Cameroon and several health zones of DRC (Likasi, Kambove, Kasenga and Malemba-Nkulu). The surveys conducted in Matadi (DRC), Kirundo (Burundi) and Batangafo (CAR) showed higher vaccination coverage than administrative estimates. Administrative coverage best approximated survey results in Malawi.

The surveys conducted in Katanga district in DRC in 2011 showed the highest coverage estimates for SIA activities (Table 3). Among these, the greatest coverage was obtained in Lubumbashi city and the lowest in Kambove, both in DRC. In Malawi, SIA coverage also varied within the country. Reported SIA administrative coverage was typically very high and often above 95%.

Surveys results showed that previous ORI conducted by MoH had the highest coverage estimate in CAR in 2011 with 79.0% [95% CI: 75.7.4-82.0%] in Batangafo and 71.0% [95% CI: 58.4-80.6%] in Kabo (Table 4). Estimated coverage was <55% in all the other locations and reached a lowest 26.7% [95% CI: 21.4-32.8%] in Maroua (Cameroon). Official administrative coverage was unavailable for these locations at this spatial scale.

While taking into account the different vaccination opportunities, a portion of children remained unvaccinated. The proportion of unvaccinated children was highest in N’Djaména in 2005 where 16.8% [95% CI: 15.0%-18.6%] of children remained unvaccinated. This figure decreased to 5.6% [95% CI: 4.6%-6.7%] in the survey conducted in 2010. However, the proportion of unvaccinated children was >5% in several sites: Moissala (Chad) (9.9% [95% CI: 5.7-16.7%]), Kabo (CAR) (8.7% [95% CI: 5.3-13.9%]), Malemba-Nkulu (DRC, 2011) (8.1% [95% CI: 5.6-11.6%]) and in Maroua city (Cameroon) (6.5% [95% CI: 4.7%-9.0%]).

After the ORI campaign, three-quarters of the children living in N’Djamena in 2005 had only received one dose of MCV (75.4% [95% CI: 73.4-77.3%]). This proportion was around one-third in Moïssala (Chad) (32.2% [95% CI: 25.9-39.2%]) and Maraoua (Cameroon) (31.9% [95% CI: 28.3-35.9%]) and between one-quarter and one-third in Kabo (29.7% [95% CI: 21.2-39.8%]), Batangafo (28.4% [95% CI: 26.2-30.8%]), and Mbuji-Mayi 27.3% [95% CI: 24.9-29.9%].

The surveys conducted in Malawi in 2010 and in DRC in 2011 where children had previous vaccination opportunities through EPI and SIA showed that ORI provided the first MCV dose to approximately 90% of children 6 to 8 months old. Among children in the EPI target group (9 to 11 months), approximately one-third received their first dose of MCV during the ORI and half received their second dose. The highest estimate for the second dose of MCV during ORI for children 9 to 11 months old was in Malawi with 55.8% [95% CI: 50.1-61.3%]. The majority of children older than 1 year but too young to be eligible for the last SIA received a second dose during the ORI. This was 78.0% of the children [95% CI: 75.4-80.4%] in Katanga province (DRC) and up to 90.9% [95% CI: 89.3-92.2%] in Malawi.

In these contexts, over half of the children eligible for the last SIA received a third dose of MCV. This was 55.8% [95% CI: 53.5-58.2% in the surveys conducted in Katanga province (DRC) and 63.4% [95% CI: 59.7-66.9%] in the surveys conducted in Malawi.

However, the proportion of children vaccinated during ORI was significantly higher among children who previously received at least one dose of MCV (Malawi and Katanga province, p < 0.001). This is best illustrated in the rural northern zone of Katanga Province (DRC) where 52.3% [95% CI:42.6%;61.9%] of children never vaccinated before the ORI, 89.9% [95% CI:86.0%;92.7%] of children who received 1 dose of MCV prior to the ORI and 91.1% [95% CI:86.8%;94.1%] of those who received 2 doses prior to the ORI were vaccinated during the campaign.