COVID-19 vaccines tweaked to better match today's omicron threat are expected to roll out in a few weeks but how much will they help? Also up in the air is who should get one -- and how soon. Pfizer and rival Moderna both asked U.S. regulators this week to authorize modified versions of their vaccine to use as fall boosters. The shot is half the original recipe and half protection against the latest two versions of omicron. The Food and Drug Administration ordered that recipe at the end of June, and now has to decide if this combination is ready.