Introduction
The Pulse Polio Immunization (PPI) program was launched in India in 1995 following the World Health Assembly resolution of 1988 for the Global Polio Eradication Initiative. It is a supplementary immunization activity (SIA) intended to complement routine immunization and achieve the goal of a polio-free world.
Scientific Rationale
The program is based on the concept of “Environmental Flooding” and the induction of high intestinal immunity:
- Mucosal Immunity: Oral Polio Vaccine (OPV) induces secretory IgA in the gut, which prevents the multiplication of wild poliovirus (WPV) and its subsequent shedding in feces.
- Environmental Flooding: Simultaneously administering OPV to all eligible children in a geographic area floods the environment with attenuated vaccine virus, displacing the WPV.
- Herd Immunity: High coverage levels ensure protection for the few individuals who may not have been vaccinated or did not seroconvert.
Implementation Strategy
The program targets all children between the ages of 0 to 5 years, regardless of their previous routine immunization status.
1. National and Sub-National Immunization Days
- National Immunization Days (NIDs): Two rounds conducted nationwide annually during the low-transmission season (usually January–March).
- Sub-National Immunization Days (SNIDs): Targeted rounds in high-risk states or districts (e.g., UP, Bihar, and urban clusters) based on surveillance data.
2. The Three-Phase Execution
- Booth Day (Day 1): Vaccination stations are set up at fixed locations (schools, booths, health centers).
- House-to-House Search (Days 2–5): Mobile teams visit every household to identify and vaccinate children who missed the booth day.
- Transit and Mobile Teams: Specialized teams vaccinate children at railway stations, bus stands, construction sites, and during transit in trains or buses.
3. Monitoring and Marking
- Finger Marking: The left little finger of the child is marked with indelible ink as a visible indicator of vaccination.
- House Marking:
- ‘P’ (Protected): All eligible children in the house have been vaccinated.
- ‘X’ (Missed): One or more eligible children in the house were absent or missed; these are revisited.
Acute Flaccid Paralysis (AFP) Surveillance
The diagnostic backbone of the program involves the “Gold Standard” AFP surveillance:
- Case Definition: Any child <15 years with acute onset of flaccid paralysis (including Guillain-Barré syndrome) or any person of any age where polio is suspected.
- Stool Sampling: Two “adequate” stool samples collected 24 hours apart within 14 days of onset of paralysis.
- Virological Analysis: Samples are sent under cold chain (+2 to +8°C) to WHO-accredited laboratories for viral isolation and “reverse transcription-polymerase chain reaction” (RT-PCR) to differentiate WPV from vaccine-derived poliovirus (VDPV).
- Environmental Surveillance: Routine testing of sewage samples from major urban centers to detect silent circulation of polioviruses.
The Polio Endgame Strategy and “The Switch”
As WPV Type 2 was declared eradicated in 2015, the risk shifted toward Circulating Vaccine-Derived Poliovirus Type 2 (cVDPV2).
- The Switch: On April 25, 2016, India executed a synchronized transition from Trivalent OPV (tOPV) to Bivalent OPV (bOPV, containing types 1 and 3).
- Inactivated Polio Vaccine (IPV): To maintain immunity against Type 2, fractional IPV (fIPV) was introduced into the National Immunization Schedule at 6 weeks, 14 weeks, and 9 months (intradermal).
Milestones and Current Status
- Last Case of WPV: Reported on January 13, 2011, in Howrah, West Bengal (WPV Type 1).
- Certification: India was certified “Polio Free” as part of the WHO South-East Asia Region on March 27, 2014.
- Current Vigilance: Maintaining high population immunity is critical to prevent the importation of WPV from remaining endemic countries (Pakistan and Afghanistan).