COVID-19 vaccines tweaked to better match today's omicron threat are expected to roll out in a few weeks, but it's still unclear how much benefit the booster shots will offer, who should get one and how soon. Pfizer and rival Moderna both asked U.S. regulators this week to authorize modified versions of their booster vaccine — shots that are half the original recipe and half offer protection against BA.4 and BA.5, the newest versions of omicron. The Food and Drug Administration ordered that recipe and now is evaluating what scientists call a "bivalent vaccine," with a decision expected soon.