Omicron looks less likely than other strains to cause severe disease. Early data suggest it is more than four times more contagious than Delta, and more able to infect those who have received a standard two-shot course of the AstraZeneca and Pfizer-BioNTech vaccines. Days after a booster shot, however, protection against infection returns to 70-75%.
But without booster shots, its rapid spread means that infection numbers could crest higher than in any of the previous waves.
A milder disease can still end up killing more people if it is more transmissible than its predecessor.
Thoughts of a Cynical Investor 
Basic common sense — law of large numbers.
A 3-dose jab is now seen as the basic definition of “fully vaxxed”, with possible 4th jab as booster.
For the next few years at least, annual covid jabs will be needed for elderly or immune compromised, just like flu jabs now.