I know the conversation has moved on, but I wanted to circle back and reiterate what others mentioned. I'm not a virologist, but I work closely with human respiratory viruses.
Viruses are extremely diverse and can cause an incredible array of diseases. You have DNA viruses and RNA viruses with different genomic structures that undergo evolution via different mechanisms. Some evolve very slowly (measles) and others very quickly (SARS-CoV-2, influenza).
Generally, there are two broad categories of evolution in viruses: genetic drift and genetic shift. Genetic drift is the slow accumulation of mutations over time, while genetic shift marks a sudden and significant change in the genome.
SARS-CoV-2 can undergo a form of genetic shift called recombination. This can occur when two different SARS-CoV-2 viruses infect the same host simultaneously and effectively trade out parts of their genomes with one another. This can result in a virus with a new genomic backbone and unique combination of mutations that can alter the new virus's properties (things like the ability to evade certain immune responses, the ability to more easily enter cells or even different tissue types). Importantly, recombination has the ability to alter the spike gene, which is what our vaccines are derived from, and depending on how dramatic that change is, it can greatly affect the efficacy of our current vaccine formulations.
Influenza has a segmented genome, and undergoes another form of genetic shift called reassortment. Similarly, if a host is simultaneously infected with two influenza viruses, they can exchange entire segments. And again, similarly, if the right segments get changed out (i.e., the hemagglutinin gene), this has the ability to render the current vaccine ineffective. This is also how pandemic influenzas are born: co-infection and reassortment between human, avian and/or swine influenza viruses.
This is why we monitor circulating viruses and compare them against the current vaccines and attempt to project which viruses next year's vaccines should be based on.
And also why you might not be as tanky as you think!
