Context: The World Health Organisation (WHO) has reported detection of poliovirus through routine surveillance of wastewater systems in five countries in the WHO European Region (Finland, Germany, Poland, Spain, and the United Kingdom) since September 2024. The presence of the virus underscores the importance of vaccination and surveillance.
Relevance of the Topic: Prelims: Key facts about Polio disease; Polio vaccines.
Major Highlights:
- A recent paper, ‘The Respiratory Route of Transmission of Virulent Polioviruses’ presents a thorough analysis of the primary transmission routes of poliovirus, particularly wild polioviruses (WPVs) and circulating vaccine-derived polioviruses (cVDPVs).
- Historically, the faecal-oral route of polio transmission was widely accepted, particularly after the introduction of the oral polio vaccine.
- The new research leans strongly towards respiratory transmission of virulent polioviruses (WPV and cVDPV) as the primary route, like other contagious infectious diseases.
- As per the research, continuing to distribute oral polio vaccine (OPV) is the wrong path to polio eradication. Vaccination with Injectable Polio Vaccine (IPV) will expedite the eradication of WPV and cVDPVs.
About poliomyelitis
- Polio is a highly infectious disease caused by a single-stranded RNA virus.
- Transmission: The poliovirus is primarily transmitted through the faecal-oral route, often via contaminated water sources or poor sanitation. It can also spread through direct contact with infected individuals.
- Types of poliovirus: Three types — wild poliovirus type 1 (WPV1), wild poliovirus type 2 (WPV2), and wild poliovirus type 3 (WPV3). Symptomatically, all these strains are identical.
- Symptoms:
- Initial symptoms are fever, fatigue, headache, vomiting, stiffness of the neck and pain in the limbs.
- The virus multiplies in the intestine and invades the nervous system and can cause total paralysis.
- One in 200 infections leads to irreversible paralysis (usually in the legs).
- Among those paralysed, 5–10% die when their breathing muscles become immobilised.
- Polio mainly affects children under 5 years of age. However, anyone of any age who is unvaccinated can contract the disease.
- Treatment: There is no cure for polio, it can only be prevented through Polio vaccines. There are two vaccines available: oral polio vaccine and inactivated polio vaccine.
Types of Polio vaccines
1. Inactivated polio vaccine (IPV):
- The first successful polio vaccine for poliovirus was made by Jonas Salk in the early 1950s.
- Salk inactivated the virus using formaldehyde and injected it into the muscles of test subjects. This inactivated polio vaccine (IPV) induced systemic immunity (relating to the blood, brain, and all other organ systems) in the subjects.
- Benefits:
- IPV contains inactivated virus particles, hence, it has no risk of causing vaccine-associated paralytic poliomyelitis (VAPP) — a rare, adverse reaction to OPV.
- Limitations:
- IPV is less potent vaccine than OPV and comparatively tougher to manufacture as it contains a chemically inactivated virus.
2. Oral polio vaccine (OPV):
- Albert Sabin developed the Oral Polio Vaccine (OPV) by using live polio virus strains that were weakened through serial cultivation in macaque cells. This process rendered the virus unfit for causing infection in humans, yet still capable of inducing an immune response.
- Because OPV contains a live virus, it is administered orally, mimicking the virus's natural route of infection.
- Benefits of OPV:
- OPV is usually preferred over IPV due to its simplicity in administration. It can be given orally without the need for syringes or specialised medical training, making it more accessible, especially in resource-limited settings.
- Additionally, it is cost-effective, making widespread immunisation campaigns feasible.
- Limitations of OPV:
- In rare cases, the weakened virus in the vaccine can mutate back to a virulent form, potentially causing the very disease it is meant to prevent.
Vaccine-derived polio:
- Vaccine-derived polio is a rare condition that occurs when the weakened (also called attenuated) strain of poliovirus used in the oral polio vaccine (OPV) mutates and regains the ability to cause paralysis.
- OPV contains a live, attenuated virus that is used for immunisation against the disease.
- The attenuated virus replicates in the intestines for a limited period and is excreted in the stool.
- This weakened virus triggers an immune response when administered, thus protecting people from the disease.
- In rare cases, the virus can mutate enough to cause the disease again and circulate in areas where either:
- immunisation is low
- immunocompromised people reside
- sanitation and hygiene are poor.
- According to the World Health Organisation (WHO), the virus is classified as “circulating” (cVDPV2) if it is detected in at least two different sources, at least two months apart, that are genetically linked, showing evidence of transmission in the community.
Key Facts:
- On World Polio Day, October 24, 2019, the WHO declared that wild poliovirus type 3 (WPV3) has been eradicated worldwide. The last case was detected in Nigeria in 2012. Wild poliovirus type 2 (WPV2) was officially declared eradicated in 2015. Thus, Only WPV1 remains in circulation.
- More than 90% of vaccine-derived poliovirus outbreaks are due to the type 2 virus present in oral polio vaccines. Vaccine-associated paralytic poliomyelitis (VAPP) constitutes 40% of cases caused by the type 2 oral polio vaccine. Many cases of VAPP from the type 3 virus also occur in countries using OPV.
- India was officially declared polio-free by the WHO in 2014. This declaration came after India successfully completed three consecutive years without reporting any new cases of wild poliovirus.
- The Indian government does not count VAPP as polio since these cases are sporadic and pose little or no threat to others, even though the number of VAPP-compatible cases showed a rising trend.
- After the global switch from trivalent (containing all three variants) to bivalent (type 1 and type 3) oral polio vaccine in 2016 to prevent any more type 2 vaccine-derived poliovirus, the number of vaccine-derived type 2 poliovirus outbreaks has only increased sharply.