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.<br /><br />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.<br /><br />The Food and Drug Administration ordered that recipe and now is evaluating what scientists call a "bivalent vaccine," with a decision expected soon.