ATLANTA - EXCLUSIVE: The Centers for Disease Control and Prevention on Tuesday will announce that the delta strain of COVID-19 is a "variant of concern" in the U.S., but that two doses of the Pfizer and Moderna vaccines are effective against it, Fox News has learned.Labeling delta a "variant of concern" is a change officials say is based on "mounting evidence" that the variant spreads more easily and causes more severe cases when compared to other variants.Biden administration officials said that they are studying the effectiveness of Johnson & Johnson's one-dose vaccine and "remain optimistic," as overseas, the AstraZeneca vaccine has demonstrated effectiveness against delta.