# Poliovirus and Polio Eradication

> Poliovirus and polio eradication for NEET-PG Microbiology: three serotypes, OPV versus IPV, AFP surveillance, vaccine-derived polio and India's certification.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/poliovirus-and-polio-eradication
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Park's Textbook of Preventive and Social Medicine
- How to cite: "Poliovirus and Polio Eradication", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/poliovirus-and-polio-eradication

## Direct answer

Poliovirus is a naked positive-sense RNA enterovirus of the family Picornaviridae, and it causes paralytic poliomyelitis in a small minority of infections. It has three serotypes, and vaccines are given either as live attenuated oral polio vaccine or as inactivated polio vaccine by injection. Eradication relies on acute flaccid paralysis surveillance, and India was certified polio-free in 2014.

## What you must remember

- **Classification:** Poliovirus is an enterovirus in Picornaviridae, a naked icosahedral virus with positive-sense single-stranded RNA.
- **Serotypes:** Types 1, 2 and 3 differ in capsid antigens, so immunity to one type does not protect against the others, and wild type 1 caused most paralytic disease.
- **Receptor:** Poliovirus binds CD155 on intestinal and neural cells and replicates first in the gut before spreading to the nervous system.
- **OPV:** The live attenuated Sabin vaccine is given orally, induces intestinal immunity and is cheap, but it can cause vaccine-associated paralytic polio.
- **IPV:** The inactivated Salk-type vaccine is given by injection and does not cause vaccine-associated paralysis, so it is used where that risk matters most.
- **Global switch:** Trivalent OPV was replaced by bivalent OPV without type 2 in 2016, after wild type 2 was declared eradicated.
- **AFP surveillance:** Acute flaccid paralysis in children under 15 is investigated with two stool samples taken at least 24 hours apart and within 14 days of onset.
- **India:** India was certified polio-free in 2014, after no wild poliovirus case had been detected since 2011.

## From a paralysed child to a certified region

Polio eradication runs on a surveillance chain that begins with a single child and ends with a regional certificate. Each link is needed, because a missed step can hide transmission.

1. **Detect the case:** A child under 15 with acute flaccid paralysis is reported promptly to surveillance and investigated without delay.
2. **Collect stool:** Two stool samples, taken at least 24 hours apart and within 14 days of onset, are sent to an accredited laboratory.
3. **Isolate and sequence:** Virus isolation in cell culture followed by genetic sequencing identifies the serotype and the lineage of the strain.
4. **Ascertain the source:** Sequencing shows whether the virus is wild or derived from oral vaccine, which determines the response needed.
5. **Respond with campaigns:** Vaccination rounds in the affected area and neighbouring districts aim to stop transmission, and India used its Pulse Polio campaigns for this purpose.
6. **Certify absence:** A region is certified free only after a prolonged period without detected wild virus, supported by surveillance of adequate quality.

A non-polio AFP rate of at least 2 per 100,000 children under 15 indicates sensitive surveillance, so the figure tells you whether a negative result can be trusted. Sequencing is the only reliable way to separate wild and vaccine-derived strains.

## Where the exam tries to catch you

Polio questions test the trade-off between OPV and IPV and the meaning of surveillance numbers.

- **Trap:** Choosing IPV for mass campaigns because it is safer — **Correct:** OPV is preferred for mass campaigns because it is cheap and induces intestinal immunity, whereas IPV avoids vaccine-associated paralysis and is used where that risk matters most.
- **Trap:** Thinking AFP surveillance needs only one stool sample — **Correct:** Two stool samples within 14 days of onset are required for sensitive detection of poliovirus.
- **Trap:** Assuming vaccine-derived poliovirus cannot cause outbreaks — **Correct:** Circulating vaccine-derived poliovirus arises in under-immunised populations, where the oral vaccine strains regain neurovirulence.

## Frequently asked questions

### How many serotypes of poliovirus exist and why does it matter?
Poliovirus has three serotypes, types 1, 2 and 3, which differ in their capsid antigens. Immunity to one serotype does not protect against the others, so vaccines must cover all the relevant types for full protection.

### Why is oral polio vaccine preferred for mass campaigns?
Oral polio vaccine is preferred for mass campaigns because it is cheap, easy to give and induces intestinal immunity that helps limit transmission. Its live attenuated virus can, however, cause vaccine-associated paralytic polio and, in low-coverage settings, circulating vaccine-derived poliovirus.

### What is the stool requirement for acute flaccid paralysis surveillance?
Two stool samples collected at least 24 hours apart, within 14 days of paralysis onset, are required for acute flaccid paralysis surveillance in children under 15. The samples go to an accredited laboratory for virus isolation and sequencing.

### When was India certified polio-free and on what basis?
India was certified polio-free in 2014 by the WHO regional certification process, after no wild poliovirus case had been detected since 2011. The certification reflected sustained surveillance and vaccination through national campaigns such as Pulse Polio.

### Why was type 2 removed from the oral polio vaccine?
Type 2 was removed from oral polio vaccine in 2016 because wild poliovirus type 2 had been declared eradicated. The switch to bivalent oral vaccine, containing types 1 and 3, also reduced the risk of vaccine-derived type 2 strains emerging.