I've noticed that Delta variant and so forth is translated using 株.
I've always understood this kanji to primarily deal with things regarding stocks, business, and so forth.
I also see from my dictionary the kanji can be used as a counter for small plants, which is the closest thing in my head so far that I can connect to then mean a variant for a virus; viruses aren't exactly plants but they are small organisms in a sense.
I also searched the kanji on this website and of its listed meanings none of them really clicked with me enough to explain how this could mean a variant of a virus. The 地位 or 評価 in definition 1 in my head could maybe logically lead to meaning variant, the status or evaluation of the virus?