Active transmission	Detection of ≥1 WPV case or of WPV isolated from ≥2 environmental samples collected >1 month apart. The end of active transmission in a previously polio- free country is the lack of isolation of WPV from environmental samples or humans for 6 months; in a country with endemic or reestablished transmission, the end of active transmission is no WPV cases/isolation for 12 months.
Endemic transmission	Circulation of indigenous WPV without interruption.
Importation event	Detection of ≥1 WPV case or ≥1 isolation from sewage in a country previously polio-free for which genomic sequences most closely match WPV actively circulating in another country (exporting WPV).
Importation outbreak	Detection of ≥1 WPV case secondary to ≥1 importation event.
Indigenous WPV	WPV that has historically been circulating in a defined geographic area of a unique genotype (>15% nucleotide difference) or cluster (>5% nucleotide difference).
Polio-free country	No evidence of indigenous WPV transmission for ≥1 year and subsequent WPV cases are determined to be due to WPV of external origin by genomic sequence analysis.
Prompt outbreak control	Status when the last identified genetically-linked WPV case is detected within 6 months of laboratory confirmation of the outbreak.
Reestablished transmission	Detection in a previously polio-free country of transmission of the same WPV lineage persisting for ≥12 months from onset of the first case following WPV importation to onset of the most recent case.