In 2010, a large outbreak of poliomyelitis with unusual 47% lethality occurred in Pointe Noire, Republic of Congo. Vaccine-mediated immunity against the outbreak virus was never investigated. A wild poliovirus 1 (WPV1) isolated from a fatal case (termed PV1-RC2010) showed a previously unknown combination of amino acid exchanges in critical antigenic site 2 (AgS2, VP1 capsid protein positions 221SAAL → 221PADL). These exchanges were also detected in an additional 11 WPV1 strains from fatal cases. PV1-RC2010 escaped neutralization by three different mAbs relevant for AgS2. Virus neutralization was tested in sera from fatal cases, who died before supplementary immunization (n = 24), Gabonese recipients of recent oral polio vaccination (n = 12), routinely vaccinated German medical students (n = 34), and German outpatients tested for antipoliovirus immunity (n = 17) on Vero, human rhabdomyosarcoma, and human epidermoid carcinoma 2 cells. Fatal poliomyelitis cases gave laboratory evidence of previous trivalent vaccination. Neutralizing antibody titers against PV1-RC2010 were significantly lower than those against the vaccine strain Sabin-1, two genetically distinct WPV1s isolated in 1965 and 2010 and two genetically distinct vaccine-derived PV strains. Of German vaccinees tested according to World Health Organization protocols, 15-29% were unprotected according to their neutralization titers (<1:8 serum dilution), even though all were protected against Sabin-1. Phylogenetic analysis of the WPV1 outbreak strains suggested a recent introduction of virus progenitors from Asia with formation of separate Angolan and Congolese lineages. Only the latter carried both critical AgS2 mutations. Antigenetically variant PVs may become relevant during the final phase of poliomyelitis eradication in populations with predominantly vaccine-derived immunity. Sustained vaccination coverage and clinical and environmental surveillance will be necessary.