Unravelling the legacy of polio: From historical origins to global eradication initiatives

26 Mar 2024

Written by Maya Cherian, Public Health Medical Officer
Reviewed by Dr Nick Walsh MD PhD(epi) BScMed(hons) MPH FAFPHM FAChAM,
Polio Egyptian Stele

Poliomyelitis, more commonly known as polio, has been circulating in the community for thousands of years. Images from 1580-1350BC show Egyptian stele portraying priest with withered leg, suggesting existence of polio for thousands of years. Since 1580BC polio has continued to be a feared disease predominantly affecting children. This was until the discovery of the inactivated polio vaccine (IPV), developed Dr Jonas Salk and first used in 1955 and the oral polio vaccine (OPV), developed by Dr Albert Sabin and first used in 1961. For medical history enthusiasts, examine the interactive timeline by Global Polio Eradication Initiative (GPEI). Despite the introduction and global scale up of coverage with polio vaccines, wildtype polio virus (WPV) remains endemic in two countries of the world and outbreaks continue to occur in areas with low vaccination rates. The COVID-19 pandemic reversed recent gains in vaccine coverage and thus set back global eradication goals.

The virus

Polio is a type of human enterovirus, with 4 subdivisions; human enterovirus A, B, C and D. Poliovirus belongs to human enterovirus C which has three serotypes, WPV 1, 2 and 3. Polio is highly infectious, transmitted via the faecal-oral route and oral-oral routes, faecal-oral being more common in areas with poor access to water and sanitation and mainly affecting children. The virus replicates in the oropharyngeal and gastrointestinal tracts resulting in viral shedding in the faeces or pharynx for multiple weeks.1

Patients acute polio can be infected with either WPV or oral polio vaccine-derived poliovirus (VDPV). There are three types of VDPD: circulating vaccine-derived poliovirus (cVDPD), immunodeficiency-related vaccine-derived poliovirus (iVDPD) and ambiguous vaccine-derived poliovirus (aVDPV), with this article focusing on circulating vaccine-derived polioviruses (cVDPV). cVDPD occurs when the OPV, a live attenuated vaccine, is onward transmitted through poor sanitation in populations with poor immunity (low vaccine coverage). Though rare, it is the lead source of new polio infections globally. IPV vaccination does not result in VDPD.

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Epidemiology

In 1988 there were 350 000 cases of polio diagnosed globally. Since then, there has been a 99% reduction of cases. WPV 2 and 3 have been declared globally eradicated, certified on 20 September 2015 and 24 October 2019 respectively. Though largely eliminated, WPV1 remains endemic in Pakistan and Afghanistan, with six cases reported in each country in 2023.2

cVDPD is rare with cVDPV outbreaks having led to 800 cases, from the delivery of 10 billion doses. In 2023 there were 625 cVDPD cases, with the majority (78%) caused by cVDPD2.

Weekly Polio Analyses Global 20240319
Clinical features and sequalae

Acute polio infection can lead to a spectrum of disease, from an asymptomatic presentation to death, commonly described as mild illness, nonparalytic polio and paralytic polio. 70% of patients present with no symptoms (asymptomatic infection), 25% experience mild symptoms, and of those that experience mild symptoms, 1-5% proceed to nonparalytic polio. Paralytic polio is the most severe form, occurring in <1% of polio virus infections. The incubation period ranges from three to six days for mild illness and to 7 to 21 days for nonparalytic and paralytic polio.

People with mild symptoms present with fever, headache, sore throat and fatigue. Nonparalytic polio causes fever, headache, vomiting and meningism in the absence of motor weakness. Nonparalytic polio usually resolves within a couple of weeks; however, some cases can progress to paralytic polio. Paralytic polio presents with acute flaccid weakness and pain, occurring after the virus infects the spinal cord. The distribution and extent of weakness ranges from a single muscle to quadriplegia and respiratory failure. Bulbar involvement can occur in up to 35% of patients, resulting in difficulty swallowing (dysphagia), difficulty speaking (dysarthria) and difficulty with secretions.

Management of acute polio is supportive with there being no antivirals currently available. Case fatality for paralytic polio is 2-5%.

Post-polio syndrome occurs in ~ 50%3 of people with a history of acute flaccid paralysis (AFP) from polio, occurring on average 35 years after acute polio. Symptoms include weakness, fatigue, pain and difficulty concentrating.

Vaccine

There are two available vaccines for routine immunisation and outbreak response for of polio, OPV and IPV. OPV is available in a number of preparations, monovalent to cover 1 serotype of polio virus, bivalent to cover serotypes 1 and 3 and trivalent documents to cover all three serotypes. The trivalent formulation was withdrawn in 2016 due to the continued threat of cVDPV2 with the use of the trivalent vaccine, despite eradication of WPV 2. IPV is a trivalent vaccine. Bivalent OPV is used in routine immunisation programs, whilst IPV is used by industrialised polio-free countries.

There are a range of advantages and disadvantages in the use of both the OPV and IPV.

Global Polio Eradication Initiative

Public health response and polio eradication

The World Health Assembly passed a resolution in 1988 to eradicate polio, which led to the establishment of the Global Polio Eradication Initiative (GPEI). The GPEI is led by national governments and six partners, with the goal of eradicating polio worldwide. The GPEI Polio Eradication Strategy 2022-2026 outlines two goals:

  1. To permanently interrupt all poliovirus transmission in endemic countries
  2. To stop cVDPV transmission and prevent outbreaks in non-endemic countries

The COVID-19 pandemic adversely impacted polio vaccination coverage, reducing global coverage unevenly across the world. This has resulted in an increase in both the absolute number of new polio cases and the number of countries reporting such cases.

Outbreak response

Polio outbreaks continue to occur globally, most recently cVDPD2 in Indonesia. There are multiple steps during a polio outbreak that must be worked through, found in the Standard Operating Procedures Responding to a Poliovirus Event or Outbreak. The vaccination response is an essential part of the polio outbreak response – a four step strategy recommended for all outbreaks: Rapid response, two large scale vaccination rounds and a mop up round. OPV is used during active polio outbreaks, as IPV does not prevent onward viral transmission.5

Surveillance

Polio surveillance enables the detection of circulating virus and new cases. Surveillance can involve AFP surveillance and environmental surveillance.

AFP surveillance involves:6

  1. Finding and reporting children with AFP
  2. Transporting stool samples for analysis
  3. Isolating the polio virus
  4. Mapping polio

Given the faecal oral transmission route, environmental surveillance of sewage or other samples allows for the detection of poliovirus in the absence of cases of polio. Systematic environmental sampling (e.g. Egypt and India) provides additional surveillance data, while ad hoc surveillance in other locations (particularly polio-free regions) can provide insight into global spread of polio.

The importance of polio eradication is realised globally, with $2.6billion in funding provided to the GPEI 2022-2026 Strategy in 2022.

The new detections of polio this year in previously polio-free countries are a stark reminder that if we do not deliver our goal of ending polio everywhere, it may resurge globally

- Dr. Tedros Adhanom Ghebreyesus, WHO Director-General. 

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Dr Maya Cherian
Public Health Medical Officer

Maya joined the NCCTRC Public Health Directorate in 2023 as a Public Health Registrar, a final year advanced trainee with the Australasian Faculty of Public Health Medicine. Maya comes to NCCTRC after working for the Office of the Chief Health Officer and the Health Statistics and Informatics branch within NT Health. Maya has been able to gain experience in the areas of Aboriginal health, public policy, communicable diseases and statistics whilst in the NT. Maya is excited to contribute to the fabric of the organisation and assist in enhancing the public health components of NCCTRC, through education, research and enhancing capabilities alongside the public health workforce on the AUSMAT roster.