A district immunisation officer in Uttar Pradesh is investigating a cluster of 3 cases of acute flaccid paralysis (AFP) in children aged 8 months to 2 years, all occurring within 4 weeks of receiving OPV. The officer suspects a potential outbreak of vaccine-derived poliovirus. What is the most appropriate investigation to confirm the serotype and differentiation between vaccine and wild-type poliovirus strains?

Explanation

## Investigation of Choice for Serotype Differentiation and VDPV Confirmation **Key Point:** Stool sample RT-PCR with sequencing and phylogenetic analysis is the definitive method to identify poliovirus serotype and differentiate vaccine-derived from wild-type strains in outbreak investigation. ### Why Sequencing and Phylogenetic Analysis? RT-PCR alone identifies poliovirus presence; sequencing adds critical information: - **Serotype identification:** Poliovirus has 3 serotypes (PV1, PV2, PV3); sequencing determines which is present - **Vaccine vs. wild-type differentiation:** VDPV has characteristic mutations in the 2C and 3D regions; phylogenetic analysis compares sequences to known vaccine and wild-type reference strains - **Outbreak source tracking:** Genetic relatedness between isolates helps determine if cases are linked ### Diagnostic Algorithm for AFP Cluster Investigation ```mermaid flowchart TD A[AFP cluster post-OPV]:::outcome --> B[Collect stool samples from all cases]:::action B --> C[RT-PCR for poliovirus]:::action C --> D{PCR positive?}:::decision D -->|Yes| E[Sequencing + phylogenetic analysis]:::action D -->|No| F[Consider non-polio causes]:::outcome E --> G{Vaccine mutations present?}:::decision G -->|Yes| H[Confirm VDPV outbreak]:::outcome G -->|No| I[Confirm wild-type poliovirus]:::outcome H --> J[Implement public health response]:::action ``` **High-Yield:** Phylogenetic analysis compares the nucleotide sequence to international databases (GenBank, WHO Polio Laboratory Network). VDPV typically shows >0.5% divergence from the vaccine strain in the VP1 region, indicating replication in the community. **Clinical Pearl:** In India's polio surveillance, all AFP cases with positive poliovirus isolation undergo sequencing at the National Polio Surveillance Project (NPSP) laboratory in Delhi. This is a mandatory step in the Global Polio Eradication Initiative (GPEI) protocol. **Mnemonic:** **SEQUENCE for SEROTYPE** — Sequencing Enables Rapid Identification of Vaccine-Derived and Epidemiological Tracking of Poliovirus. [cite:Park 26e Ch 5; WHO Polio Laboratory Manual]