Background: From January to May 2019, large measles outbreaks affected all states in Nigeria and the Federal Capital Territory. Borno state was the most affected by the measles outbreak recording 15,237 suspected cases with the state capital of Maiduguri having 1,125 confirmed cases by March 2019. Twenty-two Local Government Areas (LGAs or Districts) and 37 internally displaced persons (IDPs) camps had been affected. In response to the situation, outbreak response immunization (ORI) was conducted. In addition to conventional vaccination teams, special teams were deployed in security compromised areas, areas with migrants, and for nomadic and IDPs. Here we describe the ORI and estimated the vaccine effectiveness (VE) for measles-containing vaccine (MCV) to assess the population-level impact. A post-campaign coverage survey (PCCS) was also carried out to assess the vaccination coverage.Methods: We reviewed the outbreak response-associated immunization activities, and conducted an analysis of the surveillance and the outbreak investigation reports. We evaluated VE of MCV by applying the screening-method using data from the measles outbreak and the latest mass vaccination campaign. The PCCS was conducted after completion of the ORI through a quantitative survey assessment in 1,800 households in 12 LGAs involved in the response.Results: Of the total 15,237 reported measles cases, 2,002 cases were line-listed and investigated, and 737 were confirmed for measles as at epidemiological week 9. Of the investigated cases 67.3% (n = 1,348) were between 9 and 59 months of age. Among the 737 confirmed cases, only 9% (n = 64) stated being vaccinated with at least 1 dose of MCV. The median VE for MCV is 96.7% (95%CI: 94.7 – 98.1), and 87.3% (95%CI: 71.0 – 95.2) when vaccine was received at 9 – 11. The aggregated weighted vaccination coverage is 85.7% (95%CI: 79.6 – 90.1).Conclusion: The experience in Borno demonstrates that adequate VE can be obtained in conflict-affected areas. In complex emergency affected by measles outbreaks, health authorities may consider integration with other health strategies and the engagement of security personnel as part of the ORI activities.
