The WHO defines polio or poliomyelitis as “a highly infectious viral disease, which mainly affects young children.”
It is a crippling and potentially deadly disease that affects the nervous system.
Polio may cause disability and also is a life-threatening disease, caused by the poliovirus
The virus is transmitted by person-to-person and spread mainly through the faecal-oral route (contaminated water or food).
The virus multiplies in the intestine and then invades the nervous system and can cause paralysis.
There is no cure of the disease, only immunisation is the cure.
Polio vaccine protects children by preparing their bodies to fight the poliovirus.
There are two types of vaccines that can prevent polio:
Inactivated Poliovirus Vaccine (IPV) which is injected in the leg or arm, depending on the patient’s age.
Oral poliovirus vaccine (OPV) is given orally at birth, then primary three doses are given at 6, 10 and 14 weeks and one booster dose at 16-24 months of age.
Eradication
The last reported case of type 2 wild poliovirus was in October 1999, leading to its global eradication declaration in 2015.
However, vaccine-derived poliovirus (VDPV) outbreaks are predominantly caused by type 2 virus found in oral polio vaccines.
Around 40% of vaccine-associated paralytic poliomyelitis (VAPP) cases are caused by type 2 oral polio vaccine, despite the eradication of type 2 wild poliovirus.
The Global Polio Eradication Initiative (GPEI) has not reported VAPP cases during the 34 years of polio eradication efforts, and the Indian government does not consider VAPP as a form of polio due to its sporadic nature and minimal threat to others.
However, a 2014 report in the International Journal of Infectious Diseases revealed an increasing trend in VAPP-compatible cases in India from 1998 to 2013, outnumbering wild poliovirus cases since 2004.
The estimated incidence rate suggests that 50-100 children in India might experience VAPP each year, even though there is no official record of VAPP cases in the country.
To prevent type 2 vaccine-derived poliovirus cases, a global switch from trivalent to bivalent oral polio vaccine (OPV) took place in 2016. Additionally, one dose of inactivated polio vaccine (IPV) was introduced.
The Strategic Advisory Group of Experts on Immunization (SAGE) stated in 2015 that this switch, along with IPV introduction, would reduce and eliminate vaccine-derived type 2 poliovirus cases.
Contrary to expectations, the number of vaccine-derived type 2 poliovirus outbreaks increased significantly after the switch to bivalent OPV. In 2017, two countries reported outbreaks causing 96 VDPV type 2 cases, which increased to five outbreaks in 2018.
In 2019, 251 VDPV type 2 cases were reported from 15 countries, peaking at 1,081 cases from 26 countries in 2020. In 2021, 682 cases were reported, and 675 cases in 2022.
A type 2 novel oral polio vaccine (OPV), genetically modified to reduce the risk of reverting to neurovirulence, was authorized by WHO in 2020.
However, as of May 2023, this vaccine has already caused three type 2 VDPV cases.
The novel vaccine, used only in type 2 VDPV outbreak situations, is considered safe and effective by available evidence, but it does not address VAPP cases resulting from continued use of oral polio vaccine.
Experts have appealed to the GPEI, donors, and global opinion leaders to transition to inactivated polio vaccine (IPV) in low- and middle-income countries and phase out the oral polio vaccine to prevent further cases of polio, including VAPP.
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