Polio Elimination

a) Definition of Polio Eradication

Polio eradication is defined as the permanent reduction to zero of the worldwide incidence of infection caused by a specific agent (poliovirus) as a result of deliberate efforts. Intervention measures are no longer needed once eradication is achieved and certified.

According to the World Health Organization (WHO), the specific goals for global polio eradication are:

1. No cases of clinical poliomyelitis associated with wild poliovirus (WPV).
2. No wild poliovirus found worldwide despite intensive efforts to find it (including environmental surveillance).

The process of certification involves verifying the absence of wild poliovirus transmission from any source (Acute Flaccid Paralysis cases, community samples, and sewage samples) for at least three consecutive years in the presence of certification-standard surveillance.

b) Polio Eradication in India

India's journey toward polio eradication is considered a monumental public health milestone. Once considered the most difficult geography to clear of the virus due to high population density and poor sanitation, India has successfully interrupted transmission.

1. Historical Timeline and Milestones

• 1978: Introduction of the Expanded Program on Immunization (EPI).
• 1988: India committed to the World Health Assembly resolution for global polio eradication.
• 1995: Launch of the Pulse Polio Immunization (PPI) program, targeting children aged 0–3 years (later expanded to 0–5 years) with Oral Polio Vaccine (OPV).
• 1997: Establishment of the National Polio Surveillance Project (NPSP) in collaboration with WHO to intensify Acute Flaccid Paralysis (AFP) surveillance.
• 1999: The last case of WPV Type 2 was recorded in India.
• 2005: Introduction of Monovalent Oral Polio Vaccine (mOPV1 and later mOPV3) to enhance efficacy in high-risk areas.
• 2010: Introduction of Bivalent Oral Polio Vaccine (bOPV) in sub-national immunization days (SNIDs).
• 2010: The last case of WPV Type 3 was recorded.
• 2011 (January 13): The last case of WPV Type 1 was reported in Howrah, West Bengal.
• 2012: India was removed from the list of polio-endemic countries.
• 2014 (March 27): The South-East Asia Region, including India, was certified Polio-Free by the Regional Certification Commission (RCC).

2. Strategies Implemented

The success in India was achieved through a four-pronged strategy:

1. Routine Immunization: Maintaining high coverage with at least 3 doses of OPV and subsequent IPV introduction.
2. Supplementary Immunization Activities (SIAs): Conducting National Immunization Days (NIDs) and Sub-National Immunization Days (SNIDs) to administer OPV to all children under 5 years, regardless of previous immunization status. This aimed to break the chain of transmission rapidly.
3. AFP Surveillance: Establishing a highly sensitive surveillance system (NPSP) to detect all potential polio cases.
4. Mopping-up Campaigns: Door-to-door immunization in specific areas where the virus was known or suspected to be circulating.

3. Current Status

Since January 2011, India has reported zero wild poliovirus cases. However, the risk of importation persists from neighboring endemic countries (Afghanistan and Pakistan). Therefore, India maintains:

• High-quality AFP surveillance: Meeting global standards (Non-polio AFP rate >2/100,000).
• Environmental Surveillance: Testing sewage in multiple sites to detect silent transmission.
• Active Immunization: Continued NIDs/SNIDs and strengthening of routine immunization with IPV.

c) Immunization Issues Following Polio Eradication (The Endgame Strategy)

Following the certification of eradication, the global health community faces complex challenges regarding the cessation of OPV and the maintenance of immunity. The primary risks in the post-eradication era are Vaccine-Associated Paralytic Poliomyelitis (VAPP) and Vaccine-Derived Polioviruses (VDPVs).

1. The Risk of Oral Polio Vaccine (OPV)

While OPV was the tool of choice for eradication due to its ability to induce mucosal immunity and spread to contacts (herd effect), it carries inherent risks:

• VAPP: In rare instances (approx. 1 in 2.7 million doses), the attenuated Sabin strain in OPV can cause paralysis in the recipient or a close contact.
• VDPV: In under-immunized populations, the vaccine virus can circulate for extended periods, mutate, and regain neurovirulence, behaving like the wild virus. Circulating VDPVs (cVDPVs) have caused outbreaks in several countries.

2. The "Endgame" Strategy: Withdrawal of OPV

To eliminate the risk of VAPP and VDPV, the use of live OPV must eventually stop. This is being executed in a phased manner:

• The Switch (tOPV to bOPV): In April 2016, a globally synchronized switch was made from Trivalent OPV (containing types 1, 2, and 3) to Bivalent OPV (containing types 1 and 3). This removed the Type 2 component (Sabin 2), which was responsible for the majority of cVDPV outbreaks and VAPP cases.
• Total Withdrawal: Once WPV1 is globally eradicated, bOPV will also be withdrawn, leaving only Inactivated Polio Vaccine (IPV) for routine immunization.

3. Introduction of Inactivated Polio Vaccine (IPV)

To maintain immunity levels against poliovirus (especially Type 2 after the switch) without the risk of VAPP/VDPV, IPV has been introduced into routine immunization schedules.

• Role of IPV: IPV induces humoral immunity (IgG) preventing paralytic disease but induces limited mucosal immunity compared to OPV. It protects the individual but is less effective at preventing transmission in the community.
• Supply Constraints and Fractional Dosing: Due to a global shortage of IPV during the switch, countries like India adopted the use of fractional dose IPV (fIPV). Administered intradermally (0.1 ml, 1/5th of the full dose), fIPV has been shown to be immunogenic and dose-sparing.
• Current Schedule in India: As of January 1, 2023, the schedule includes three doses of fIPV at 6 weeks, 14 weeks, and 9 months, alongside bOPV.

4. Managing Risks in the Post-Eradication Era

• Outbreak Response: If cVDPV2 outbreaks occur after the cessation of tOPV, monovalent OPV2 (mOPV2) or the Novel Oral Polio Vaccine Type 2 (nOPV2) is used. nOPV2 is genetically more stable and less likely to revert to neurovirulence than mOPV2.
• Containment: Strict laboratory containment of all wild and vaccine polioviruses is essential to prevent accidental release and re-establishment of transmission.
• Surveillance: Enhanced surveillance, including environmental sampling and surveillance for immunodeficient excretors (iVDPV surveillance), is critical to detect any re-emergence immediately.

5. Integration and Transition

As dedicated polio funding decreases, the infrastructure built for polio (surveillance networks, cold chain, trained personnel) must be integrated into broader national health systems to sustain immunization coverage against other vaccine-preventable diseases.