## **Core Concept**
The national switch day for polio vaccine refers to the date when India switched from the trivalent oral poliovirus vaccine (tOPV) to the bivalent oral poliovirus vaccine (bOPV) as part of its strategy to eradicate polio. This switch was a significant step in the global effort to stop the transmission of poliovirus.

## **Why the Correct Answer is Right**
The correct answer, , marks the date when India made this critical switch. The switch to bOPV was aimed at reducing the risk of vaccine-derived poliovirus type 2 (cVDPV2) cases, which were being reported despite the overall decline in polio cases. By using bOPV, which protects against poliovirus types 1 and 3 but not type 2, the risk of cVDPV2 was expected to decrease significantly.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not correspond with the known date of the vaccine switch in India.
- **Option B:** This option is incorrect for the same reason; it does not align with the historical data regarding the switch from tOPV to bOPV in India.
- **Option D:** This option is incorrect as it also does not match the documented date of the national switch day for the polio vaccine in India.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the switch from tOPV to bOPV was a strategic move to enhance the eradication of poliovirus, particularly in regions where type 2 poliovirus had been eliminated. This switch highlights the adaptive strategies used in vaccine programs to overcome challenges in achieving and maintaining disease eradication.

## **Correct Answer:** . 2016