Epidemiology

Poliovirus infection has been eliminated from the western hemisphere, the last case occurring in Peru in 1991.[2] The last case of wild-type poliovirus infection in the US was in an Amish community in 1979, but travel-related acquisition of poliovirus infection may occur periodically.[3]​ However, in a fully immunized community, with sanitary practices, without active circulating wild-type virus, the danger of poliovirus transmission is minimal. In August 2020, the World Health Organization (WHO) African Region was declared free of wild poliovirus.[4]

The epidemiology of poliomyelitis worldwide can be considered as three separate groups: outbreaks of poliomyelitis caused by wild poliovirus, either endemic or imported; sporadic cases of vaccine-associated paralytic poliomyelitis (VAPP); and outbreaks of vaccine-derived poliomyelitis caused by circulating vaccine derived poliovirus (cVDPV). There are three strains of wild poliovirus: type 1, type 2, and type 3. Wild poliovirus type 2 and type 3 have been certified by WHO as globally eradicated and wild poliovirus type 1 (WPV1) is now endemic only in Afghanistan and Pakistan.[5]​​ Until September 2024, the number of WPV1 confirmed cases reported to WHO was 18 in Afghanistan and 17 in Pakistan compared to 6 in Afghanistan and 6 in Pakistan reported in 2023.[5]​ In 2021, WPV1 was reported in a 3-year-old girl in Malawi, the first case in Africa in over 5 years. Analysis shows that the strain detected in Malawi was genetically linked to WPV1 that was circulating in Pakistan in 2019-2020.[6]​​[7]​​ In 2022, 8 confirmed cases of WPV1 were reported in Mozambique (which borders Malawi); analysis confirms genetic linkage to the isolate detected in Malawi.​[8]​ During 2023 no cases were reported from either Malawi or Mozambique.[5]​​ Currently, most outbreaks of cVDPV are caused by type 2 cVDPV and most cVDPV cases are reported from the African continent (475 of 491 in 2023).[9][10]​​​​ Between August 2021 and July 2023, seven cVDPV2 outbreaks originating from type 2 novel oral poliovirus vaccines (nOPV2) were identified in six African countries, based on 61 cases of paralysis and 39 environmental surveillance (sewage) samples.[11]​ In 2022, the isolation of type 2 VDPV in environmental samples in London, UK was confirmed as part of routine surveillance. Type 2 VDPV was also detected in environmental samples in the US, Canada, and Israel, with one case of paralytic polio reported in New York and one in Israel.[10][12][13][14]​​​​​ Genetic analysis suggests that the isolates detected had a common origin, and no isolates have been reported from these countries during 2023.[10][12][15][16]​​​​​​​​ VDPV arises from oral attenuated poliovirus vaccine (OPV), which has not been used in the US since 2000 or the UK since 2004 and so it is likely that the recently-detected VDPV originated from an individual who arrived from a country where OPV has been used for supplementary immunization campaigns.[15]​​[17]

In 2023, the WHO continued to report that international spread of poliovirus remains a Public Health Emergency of International Concern.[18]​​

In temperate climates, poliovirus infections are most common in the summer and autumn months.[19]​ In tropical climates, there is less of a seasonal pattern, but infections may be more common during rainy seasons. Poliovirus infection mainly affects children aged under 5, with the vast majority under 36 months, particularly those who are not immunized.[20]

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