Background: The European Region, certified polio-free in 2002, remains at risk of wild poliovirus reintroduction and emergence of circulating vaccine-derived polioviruses (cVDPV) until global polio eradication is achieved, as demonstrated by the cVDPV1 outbreak in Ukraine in 2015.

Methods: We reviewed epidemiologic, clinical and virology data on cVDPV cases, surveillance and immunization coverage data, and reports of outbreak-related surveys, country missions, and expert group meetings.

Results: In Ukraine, 3-dose polio vaccine coverage declined from 91% in 2008 to 15% by mid-2015. In summer, 2015, two unrelated children from Zakarpattya province were paralyzed by a highly divergent cVDPV1. The isolates were 20 and 26 nucleotide divergent from prototype Sabin strain (with 18 identical mutations) consistent with their common origin and ∼2-year evolution. Outbreak response recommendations developed with international partner support included conducting three nationwide supplementary immunization activities (SIAs) with tOPV, strengthening surveillance and implementing communication interventions. SIAs were conducted during October 2015-February 2016 (officially reported coverage, round 1-64.4%, round 2-71.7%, and round 3-80.7%). Substantial challenges to outbreak response included lack of high-level support, resistance to OPV use, low perceived risk of polio, widespread vaccine hesitancy, anti-vaccine media environment, economic crisis and military conflict. Communication activities improved caregiver awareness of polio and confidence in vaccination. Surveillance was enhanced but did not consistently meet applicable performance standards. Post-outbreak assessments concluded that cVDPV1 transmission in Ukraine has likely stopped following the response, but significant gaps in population immunity and surveillance remained.

Conclusions: Chronic under-vaccination in Ukraine resulted in the accumulation of children susceptible to polioviruses and created favorable conditions for VDPV1 emergence and circulation, leading to the outbreak. Until programmatic gaps in immunization and surveillance are addressed, Ukraine will remain at high-risk for VDPV emergence and circulation, as well as at risk for other vaccine-preventable diseases.