Description of a large and deadly measles epidemic, Yakusu health zone, Democratic Republic of Congo, 2018-2019: A retrospective study

Background A large measles outbreak occurred in Yakusu Health Zone, Democratic Republic of the Congo, between Jan 2018 and June 2019. We describe the extent of the outbreak and it’s impact on the population. Methods We conducted a retrospective household survey to describe the attack rate and various mortality measures, and to assess vaccination coverage both before and after a mass reactive vaccination campaign. We estimated the true outbreak size by extrapolating the attack rate and case fatality ratios to the population of Yakusu. We conducted geospatial analyses to describe the distribution of measles attack rate and vaccination coverage. Results 8,968 individuals were included in the survey, of whom 1,390 (15.5%, 95%CI 13.2-17.8) reported to have had measles during the recall period. The outbreak size was estimated as 22,068 (95% CI 18,980-25,553). Overall case fatality ratio was 2.5% [95% CI 1.7-3.4]; measles-specific proportional mortality was 15.3% [95%CI 10.9-19.8]; and the all-cause mortality rate was 0.72 per 10,000 person-days [95% CI 0.61-0.82]. For children aged under 5 years, the overall attack rate was 45.7% [95% CI 39.6-51.9]; case fatality ratio was 4.9% [95% CI 3.4-6.3]; measles-specific proportional mortality was 35.2% [95%CI 26.0-44.5]; and all-cause mortality rate was 1.51 per 10,000 person-days [95% CI 1.19-1.84]. Children aged under 1 year had the greatest risk of death. Two-thirds of the target population had been vaccinated during routine Expanded Programme on Immunization (EPI, 67.9% [95%CI 62.2-73.6]) and during the campaign (67.7% [95%CI 61.6-73.8]). Coverage according to either EPI or campaign was 82.1% [95%CI 78.0-86.1]). There was a high degree of spatial overlap in vaccination coverage by EPI and campaign, with pockets of low coverage throughout Yakusu, within which the epidemic propagated. Conclusions This was a high-impact epidemic, both in terms of attack rate and case fatality, with an elevated risk of dying for at least 90 days post-onset of disease. Reactive vaccination activities were effective at the individual level, but were implemented late, did not reach intended coverage, and were insufficiently targeted to ensure that previously unvaccinated children received vaccination, thereby leaving pockets of unvaccinated children among whom the outbreak appeared to persist.