What molecular method is used to determine whether a poliovirus isolate is wild-type or vaccine-derived?

• A. Whole genome sequencing
• B. VP1 gene sequencing and comparison to reference vaccine strains
• C. Serotyping by neutralization
• D. Electron microscopy imaging

Answer: (B)

The key idea is using the VP1 gene to tell apart wild-type poliovirus from vaccine-derived strains. VP1 encodes a major capsid protein and accumulates mutations as the virus replicates in humans, so its sequence provides a clear record of lineage. By sequencing the VP1 region and comparing it to the Sabin vaccine strain sequences, you can gauge how far the isolate has diverged from the vaccine origins. If the VP1 sequence is sufficiently different and clusters with known wild-type lineages, the virus is wild-type. If it shows the specific level of divergence from Sabin that defines a vaccine-derived variant, it is classified as vaccine-derived. This approach is preferred because VP1 gives the most practical, informative marker for poliovirus lineage and surveillance, whereas methods like serotyping by neutralization or electron microscopy don’t distinguish vaccine-derived from wild-type at the genetic level, and while whole-genome sequencing can provide broader data, VP1 sequencing is the standard, efficient method for this determination.