We have seen plenty of viral variants, some with Spike genes of sufficient interest to merit a Greek letter. Spike is the virus’s entry weapon, and the bit of the virus that’s targeted by vaccines. If your antibodies block Spike, you block the virus – and if Spike has mutated, it may have become better at dodging those antibodies. Delta has been the most vicious variant so...
Since Delta, every variant from Epsilon to Kappa has been downgraded, with Lambda and Mu still designated as merely 'Variants of Interest'. If the mighty Delta could be crushed by the first-generation vaccines, albeit in three doses, what hope for the Nu variant when it came along? But Nu was too much like the English ‘new’, and Xi too much like the president of the People’s Republic of China; so we arrive at Omicron.