As you know, new variants of old viruses are named by using a variable letter. Virus.a, Virus.b, Virus.c...etc.
When we have more than 26 variants of a virus, we run out of letters. Then we roll over from Virus.z to Virus.aa, Virus.ab, Virus.ac etc.
For some virus families, even this is not enough. They've become so large (over 700 members) that we've ended up to variant zz. When we, of course, roll over start over with Virus.aaa, Virus.aab, Virus.aac etc.
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For reference, here are the virus families that have already wrapped to "three digits", ie. over variant letter ".aaa".
