Context:

A two-year-old child in Tikrikilla, Meghalaya, has been infected with vaccine-derived polio. This is not a case of wild poliovirus, but an infection that presents in some people with low immunity, the Union Health Ministry said on Tuesday, August 20.

What is Vaccine-Derived Polio?

Vaccine-derived polio is a rare condition that occurs when the weakened (attenuated) strain of poliovirus used in the oral polio vaccine (OPV) mutates and regains the ability to cause paralysis. The OPV contains a live but weakened virus that stimulates an immune response, thereby immunising people against poliovirus. The attenuated virus typically replicates in the intestines and is excreted through stool.

In rare cases, the virus can mutate and regain its virulence, causing the disease it was meant to prevent. This typically occurs in areas with low immunisation coverage, poor sanitation, or in immunocompromised individuals. When this mutated virus begins circulating within the community, it is referred to as circulating vaccine-derived poliovirus (cVDPV). According to the World Health Organization (WHO), a virus is classified as cVDPV2 if it is detected in at least two different sources, over two months apart, and shows genetic linkage, confirming community transmission.

Types of Poliovirus

Polioviruses are enteroviruses transmitted primarily via the faecal-oral route. There are three types of polioviruses:

• Wild Poliovirus Type 1 (WPV1)
• Wild Poliovirus Type 2 (WPV2)
• Wild Poliovirus Type 3 (WPV3)

Though symptomatically identical, these strains vary in their distribution and prevalence. The widespread use of vaccines has helped eliminate some strains globally, but vaccine-derived polio remains a concern in some regions.

Symptoms and Transmission

Poliovirus usually leads to acute, short-term infections, presenting symptoms that vary from mild to severe. Common signs include fatigue, fever, headaches, vomiting, diarrhea, constipation, sore throat, neck stiffness, tingling sensations in the arms and legs, severe headaches, and light sensitivity (photophobia). The virus primarily targets the gastrointestinal system and is expelled through feces, enabling transmission in unsanitary environments.

Infected individuals remain contagious for up to two weeks, emphasizing the need for early detection and isolation to prevent outbreaks. It's noteworthy that humans are the sole reservoir for poliovirus, with no animal or insect vectors involved in its spread. As a result, comprehensive human immunization strategies have the potential to completely eradicate the virus, as has been largely achieved with wild poliovirus.

The Evolution of Polio Vaccines

The First Polio Vaccine

The first successful polio vaccine was developed by Jonas Salk in the early 1950s. Known as the inactivated polio vaccine (IPV), this vaccine used formaldehyde to inactivate the virus, preventing it from causing infection. The vaccine induced systemic immunity and protected individuals from polio.

Sabin’s Oral Polio Vaccine (OPV)

Albert Sabin later developed an oral polio vaccine using live, attenuated poliovirus strains. These strains were weakened by repeated growth in macaque cells, making them less likely to cause infection. OPV is administered orally, triggering a natural immune response without the need for injections. OPV is widely preferred because of its ease of administration, affordability, and ability to induce both systemic and local (intestinal) immunity.

The Risks of OPV

Vaccine-Derived Poliovirus (VDPV)

Despite its advantages, OPV carries a small risk of causing vaccine-derived poliovirus (VDPV). The live, attenuated virus can sometimes mutate and revert to a more virulent form, particularly in areas with low vaccine coverage or poor sanitation. This can lead to outbreaks of vaccine-derived polio. In contrast, IPV contains inactivated virus particles, which cannot cause the disease but is more expensive and difficult to manufacture.

Vaccine-Associated Paralytic Poliomyelitis (VAPP)

Another risk associated with OPV is vaccine-associated paralytic poliomyelitis (VAPP), a rare adverse reaction where the weakened virus in the vaccine causes paralysis. VAPP is a significant concern in areas where OPV is widely used, particularly with type 2 poliovirus, which has been responsible for over 90% of vaccine-derived poliovirus outbreaks. After the global switch from trivalent (containing all three poliovirus types) to bivalent OPV in 2016, intended to prevent type 2 VDPV, the number of type 2 VDPV cases has paradoxically increased.

Global Progress and New Strategies

Eradication of Wild Poliovirus

On World Polio Day, October 24, 2019, the WHO declared wild poliovirus type 3 eradicated worldwide, with the last case detected in Nigeria in 2012. Wild poliovirus type 2 was officially eradicated in 2015. However, outbreaks of vaccine-derived poliovirus continue to occur, primarily due to the type 2 virus in OPV.

New Developments in Polio Vaccination

To combat the rising number of type 2 VDPV cases, the WHO authorized a genetically modified type 2 novel oral polio vaccine (nOPV2) under Emergency Use Listing in November 2020. This new vaccine, first used in March 2021 and prequalified by the WHO in December 2023, is designed to be less likely to revert to a virulent form, reducing the risk of vaccine-derived polio outbreaks.

India’s Efforts

• Pulse Polio Programme: A nationwide campaign aimed at immunizing all children under the age of five against polio, which has significantly reduced polio cases in the country.
• Intensified Mission Indradhanush 2.0: This mission focuses on ensuring full immunization coverage in areas with low vaccine penetration, particularly among marginalized populations.
• Universal Immunization Programme (UIP): Launched in 1985 as an evolution of the Expanded Programme of Immunization (EPI), the Universal Immunization Programme (UIP) was established with the goal of rapidly increasing immunization coverage across India. The programme emphasizes enhancing the quality of vaccination services while ensuring a reliable cold chain system to preserve vaccines at the health facility level. Additionally, the UIP includes a district-wise monitoring system to track the performance of immunization efforts. One of its key objectives is to promote self-sufficiency in vaccine production within the country, ensuring sustainable progress in public health immunization.

Conclusion

Vaccine-derived polio remains a rare but significant challenge in global public health efforts to eradicate poliovirus. While OPV has been instrumental in reducing polio cases worldwide, it carries the risk of vaccine-derived outbreaks, particularly in regions with low immunization coverage. Gradually phasing out the use of Oral Polio Vaccine (OPV) and replacing it with Inactivated Polio Vaccine (IPV) helps reduce the risk of VDPV. Continued vigilance, improved sanitation, and the introduction of new vaccines such as nOPV2 are critical to fully eliminating this disease.

Source: The Hindu