Polio Risk Assessment: Regional Implications for the Americas, 25 April 2023 – World

Background information

Hazard assessment

Poliovirus is a human enterovirus with three serotypes, types 1, 2 and 3. However, after the polio vaccine was introduced, only wild poliovirus (WPV) type 1 remained circulating. His last WPV type 2 (WPV2) was isolated in 1999 and declared eradicated in 2015, while his last WPV type 3 (WPV3) was isolated in 2012 and declared eradicated in 2019. I was. Polio is transmitted through the fecal-oral route, enters the body through the mouth, and multiplies in the intestine. Infected people release the poliovirus into the environment for weeks, where it spreads rapidly in areas with poor sanitation (1-4).

The incubation period is 7 to 21 days (range – 40 days). Poliovirus infection causes mild illness with mild symptoms in infections such as the central nervous system and can cause paralysis. Approximately 90% of infections are asymptomatic or present with nonspecific fever. Other symptoms include aseptic meningitis, fever, malaise, headache, nausea, and vomiting. If the disease progresses with severe clinical symptoms, severe muscle pain, neck and back stiffness with flaccid paralysis may occur. Paralysis of respiratory muscles can be life-threatening. Some improvement in paralysis may be seen during recovery, but paralysis that persists after 60 days may be permanent (1-4).

All cases of AFP in children under 15 years of age should be reported and stool samples tested for virus isolation by cell culture. Once positive segregation is obtained, perform intratypic differentiation by quantitative reverse transcription-polymerase chain reaction (RT-qPCR). These assays allow the first characterization of Sabin-like or non-Sabin-like polioviruses. If vaccine-derived poliovirus (VDPV) is suspected, her VP1 region sequencing test of the viral genome allows genetic characterization of the virus and is the final confirmatory test (1–4).

Diagnosis of paralytic poliomyelitis is based on (i) clinical course, (ii) virological examination, and (iii) residual neuropathy (1-4) 60 days after onset of symptoms.

There are two types of vaccines commercially available for routine immunization: oral poliovirus (OPV) vaccine and inactivated poliovirus (IPV) injectable vaccine. In rare cases, adverse events associated with OPV can lead to cases of vaccine-associated paralytic polio (VAPP), which can occur in vaccinated individuals or their contacts, and the emergence of vaccine-derived poliovirus. There is a nature. The incidence of VAPP is estimated at 2-4 cases per million live births per year in countries with OPV (1).

At very low vaccination coverage, the live attenuated virus (Sabin virus) from the OPV vaccine reproduces the neurovirulence and transmission characteristics of wild poliovirus (WPV) through prolonged replication in individuals or communities. may obtain. Genetic mutations can lead to VDPV, causing isolated cases and outbreaks of paralytic polio. VDPV is a genetically distinct form of the original Sabin vaccine virus, traditionally defined by >1% genetic divergence (or >10 nucleotide changes). [nt]) in the VP1 region of the viral genome, >0.6% (or >6 nt change) for PV1 and PV3 and >0.6% for PV2 (1).

Vaccine-derived poliovirus (VDPV) falls into three categories: (1) cVDPV, circulating VDPV, is VDPV that exhibits human-to-human transmission in the community based on the detection of genetically related viruses in human and/or environmental samples. (2) iVDPV, immunodeficiency-associated VDPV, is VDPV from individuals with evidence of primary immunodeficiency; Unlike immunocompetent individuals who shed the vaccine virus for a limited period, rarely, patients with primary immunodeficiency may shed a genetically distinct vaccine virus for a long time after receiving OPV. (3) aVDPV, an ambiguous VDPV, is a polio VDPV whose VP1 sequence is not genetically related to other his VDPV sequences previously identified and, if the virus originates from an individual, a primary immunodeficiency No proof. The VDPV sequence is classified as ambiguous based on laboratory results, epidemiological studies, and communication with field teams, technical experts, and laboratory staff at WHO headquarters and his WHO regional office. Isolates may come from known immunocompromised individuals or from environmental samples without evidence of circulation (5).