Pulse Polio Immunization

Introduction

Pulse Polio Immunization (PPI) is the largest single-day public health project in the world. It is a mass immunization campaign established by the Government of India in 1995 to eradicate poliomyelitis from the country. It was launched in pursuance of the World Health Assembly's resolution (1988) to eradicate polio globally. The initiative successfully eliminated the wild poliovirus from India, leading to the World Health Organization (WHO) certifying India as "Polio-free" in March 2014. However, the program continues in a modified form to maintain immunity and prevent the re-emergence or importation of the virus.

Concept and Rationale

The term "Pulse" refers to the simultaneous administration of oral polio vaccine (OPV) to the entire target population (children under 5 years of age) on a single day, regardless of their previous immunization status.

• Objective: To break the chain of transmission of the wild poliovirus.
• Mechanism:
  • Saturation: By immunizing every child simultaneously, the vaccine virus floods the environment and the intestines of the child population.
  • Displacement: The vaccine virus competes with and displaces the wild poliovirus in the environment and the host.
  • Gut Immunity: OPV induces robust mucosal (intestinal) immunity (IgA), which prevents the multiplication of the wild virus in the gut and its subsequent excretion in stool, thereby halting fecal-oral transmission.
  • Herd Immunity: High coverage ensures that even unvaccinated or susceptible individuals are indirectly protected because the virus has no hosts to infect.

Strategies of the Eradication Initiative

The PPI program operates as one of the four pillars of the Global Polio Eradication Initiative (GPEI). The four pillars are:

1. High Routine Immunization Coverage: Ensuring infants receive the primary doses of OPV/IPV during the first year of life.
2. Supplementary Immunization Activities (SIAs): These are the Pulse Polio campaigns.
   • National Immunization Days (NIDs): Conducted across the entire country, typically twice a year during the low transmission season (winter months).
   • Sub-National Immunization Days (SNIDs): Conducted in high-risk states or areas (e.g., Uttar Pradesh, Bihar) to maintain high immunity levels.
3. Acute Flaccid Paralysis (AFP) Surveillance: To detect and investigate every potential case of polio.
4. Mopping-up Immunization: Intensive door-to-door immunization in specific geographic areas where the virus is known or suspected to be circulating.

Operational Methodology

A standard NID or SNID involves a rigorous operational cycle:

1. Booth Day (Day 1): Vaccine booths are set up at accessible locations (schools, health centers, transit points). Parents bring children <5 years to receive two drops of OPV.
2. House-to-House Activity (Days 2–3): Teams of health workers visit every house to identify and vaccinate children who missed the booth activity.
3. Finger Marking: To prevent duplication and ensure coverage, the little finger of the child's left hand is marked with indelible ink.
4. Transit Teams: Vaccination teams are deployed at railway stations, bus stands, and highways to vaccinate children in transit.

Evolution of Vaccines Used

The choice of vaccine used in PPI has evolved based on the epidemiological landscape.

• Trivalent OPV (tOPV): Initially, tOPV containing serotypes 1, 2, and 3 was used. This successfully eradicated Wild Poliovirus Type 2 (WPV2) globally in 1999.
• Monovalent (mOPV) and Bivalent (bOPV): To tackle the remaining WPV1 and WPV3 more effectively, mOPV1 and mOPV3 were used in high-risk areas. Later, bOPV (containing types 1 and 3) became the vaccine of choice for campaigns.
• The "Switch" (April 2016): As part of the Polio Endgame Strategy, there was a globally synchronized switch from tOPV to bOPV.
  • Rationale: Since WPV2 was eradicated, the continued use of the type 2 component in tOPV posed a risk of Vaccine-Associated Paralytic Poliomyelitis (VAPP) and the emergence of Circulating Vaccine-Derived Polioviruses (cVDPVs) type 2. Removing type 2 from the oral vaccine eliminates this risk.
• Inactivated Polio Vaccine (IPV): To maintain immunity against type 2 after withdrawing it from OPV, at least one dose of IPV was introduced into the routine immunization schedule (at 14 weeks). In India, due to global supply constraints, two fractional doses (0.1 mL intradermally) are often used at 6 and 14 weeks.

Surveillance

A robust surveillance system is the backbone of the eradication effort.

• AFP Surveillance: All cases of acute onset flaccid paralysis in children <15 years are reported immediately. Two stool samples are collected 24–48 hours apart within 14 days of onset to check for poliovirus.
• Environmental Surveillance: Sewage samples from sentinel sites are tested to detect the presence of poliovirus in the community, providing an early warning system even before clinical cases appear.

Current Challenges and The Endgame

Although India is polio-free, the risk of resurgence remains due to:

1. Importation: Wild poliovirus is still endemic in neighboring countries like Pakistan and Afghanistan.
2. Vaccine Derived Polio Virus (VDPV): In areas with low immunization coverage, the attenuated vaccine virus can mutate and regain neurovirulence (cVDPV), causing paralysis. This is particularly problematic with type 2 virus, necessitating the switch to bOPV and IPV.
3. VAPP: A rare adverse event (approx. 1 in 1 million doses) where the live vaccine virus causes paralysis in the recipient or a contact. The ultimate goal is to stop all OPV use and rely solely on IPV once wild polio transmission stops globally.