SSRNA viruses just randomly mutate, and some of the mutations work around whatever vaccine is attempted, subsequently self selecting by surviving, and propagates merrily away.
One of nature's many ways of thinning the herd and strengthening the survivors.
23 posted on 05/12/2020 8:10:42 AM PDT by JimRed
(TERM LIMITS, NOW! Build the Wall Faster! TRUTH is the new HATE SPEECH.)