Polio immunity and the impact of mass immunization campaigns in the Democratic Republic of the Congo

Abstract

Background: In order to prevent outbreaks from wild and vaccine-derived poliovirus, maintenance of population immunity in non-endemic countries is critical.

Methods: We estimated population seroprevalence using dried blood spots collected from 4893 children 6-59months olds in the 2013-2014 Demographic and Health Survey in the Democratic Republic of the Congo (DRC).

Results: Population immunity was 81%, 90%, and 70% for poliovirus types 1, 2, and 3, respectively. Among 6-59-month-old children, 78% reported at least one dose of polio in routine immunization, while only 15% had three doses documented on vaccination cards. All children in the study had been eligible for at least two trivalent oral polio vaccine campaigns at the time of enrollment; additional immunization campaigns seroconverted 5.0%, 14%, and 5.5% of non-immune children per-campaign for types 1, 2, and 3, respectively, averaged over relevant campaigns for each serotype.

Conclusions: Overall polio immunity was high at the time of the study, though pockets of low immunity cannot be ruled out. The DRC still relies on supplementary immunization campaigns, and this report stresses the importance of the quality and coverage of those campaigns over their quantity, as well as the importance of routine immunization.

Keywords: Democratic Republic of the Congo; Immunization; Mass vaccination; Poliomyelitis; Seroprevalence.

Fig. 1

Epidemiology and polio vaccination program history in the DRC. Top panel: the geographic distribution of cases in the periods 2006–2009 and 2010–2012; points are placed randomly in the district where a child was present two weeks prior to the onset of paralysis. Bottom panel: polio AFP case count by three-month bins; above the case counts are bars representing supplemental immunization activities. The grey band shows the period in which the DHS survey was conducted.

Fig. 2

Sample size resulting from applying inclusion criteria and removing missing data.

Fig. 3

Seroprevalence by demographic characteristics.

Fig. 4

Seroprevalence and routine immunization coverage, by province.

Fig. 5

Seroprevalence by routine immunization status and SIA eligibility. Note, fewer SIAs with type-2 containing vaccine were conducted than for types 1 and 3. Thus, estimates for type 2 seroprevalence are not available for children who have experienced 11 or more SIAs.

Fig. 6

Seroprevalence by SIA eligibility, by stratified by routine immunization status. Left panel: those who report no DTP routine immunization. Right panel: those with documented full DTP immunization.

Supplementary Fig. S1