A 6-month-old infant from rural Maharashtra presents to the immunisation clinic with a history of incomplete vaccination. The mother reports that the child received BCG and OPV-0 at birth, but missed the 6-week vaccines due to illness. On examination, the child is afebrile and clinically well. The health worker suspects possible adverse event following immunisation (AEFI) from a previous dose. Which investigation is most appropriate to confirm vaccine strain poliovirus in case of vaccine-derived poliovirus (VDPV) suspected paralysis?

Explanation

## Investigation of Choice for Suspected Vaccine-Derived Poliovirus (VDPV) **Key Point:** Stool sample for viral isolation and RT-PCR is the gold standard for detecting poliovirus, including vaccine-derived strains, in suspected AEFI cases. ### Why Stool Sampling? Poliovirus replicates primarily in the gastrointestinal tract and is shed in faeces for 3–6 weeks after infection or vaccination. This makes stool the ideal specimen for: - Viral isolation in cell culture (RD or L20B cells) - RT-PCR amplification and sequencing to differentiate vaccine strain from wild-type - Confirmation of VDPV in AEFI surveillance ### Investigation Algorithm for Suspected VDPV Paralysis ```mermaid flowchart TD A[Suspected VDPV paralysis post-vaccination]:::outcome --> B{Specimen type?}:::decision B -->|Stool| C[Viral isolation + RT-PCR]:::action B -->|CSF| D[Low sensitivity for poliovirus]:::outcome B -->|Serum| E[Antibody only - no differentiation]:::outcome C --> F[Identify vaccine vs wild-type strain]:::action F --> G[Confirm VDPV diagnosis]:::outcome ``` **High-Yield:** RT-PCR with sequencing can differentiate vaccine strain (attenuated) from wild-type poliovirus within 24–48 hours, enabling rapid AEFI classification and public health response. **Clinical Pearl:** In India's polio surveillance system, stool samples from suspected cases are sent to the National Polio Surveillance Project (NPSP) laboratory for confirmation. Two stool samples collected 24 hours apart improve sensitivity to >95%. **Mnemonic:** **STOOL for POLIOVIRUS** — Stool Testing Offers Optimal Identification of Poliovirus (Isolation and RT-PCR). [cite:Park 26e Ch 5]