It's just that the more optimal versions of the virus will spread more effectively out competing other variants that might be too deadly or not transmissible enough omicron came outta left field like she stomped on the scene, this baby has 30 mutations in the spike protein and has evolved to infect people who are immune through vaccination or previous infection to delta, but as gains in transmissibility slow, it needs to figure out ways to continue to overcome immune responses, which is probably what's happening right now, experiments and lab sequencing of variants circulating right now have identified a bunch of mutations in spike proteins that weaken your body's immune response which has been boosted by vaccination or prior infection.