The magical potato vaccine detox! Because obviously, tubers have evolved specifically to extract "toxins" from injection sites through some kind of starchy osmosis. Next up: using bananas as WiFi boosters and cucumbers to absorb student loan debt.
The real genius is the second comment - sometimes it's easier to let people believe potatoes have supernatural powers than to explain basic immunology. When your scientific explanation gets trumped by root vegetables, you know you've reached peak internet.