FILE - Pfizer COVID-19 vaccines sit ready for use at a Dallas County Health and Human Services drive up vaccine site in Mesquite, Texas, Nov. 30, 2021. While all eyes are on the new and little-understood omicron variant, the delta form of the coronavirus isn't finished wreaking havoc in the U.S. There is much that is unknown about omicron, including whether it is more contagious than previous versions, makes people sicker or more easily thwarts the vaccine or breaks through the immunity that people get from a bout of COVID-19. (AP Photo/LM Otero, File)