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Updated (Bivalent) Boosters
The updated (bivalent) boosters are called “bivalent” because they protect against both the original virus that causes COVID-19 and the Omicron variant BA.4 and BA.5.

Previous boosters are called “monovalent” because they were designed to protect against the original virus that causes COVID-19. 

They also provide some protection against Omicron, but not as much as the updated (bivalent) boosters.

https://www.cdc.gov/coronavirus/2019-nco...-date.html