delta variant makes up 6% of samples tested in the u.s., but dr. fauci warning the u.s. is not in the clear. >> we cannot let that happen in the united states. if you've had your first dose, make sure you get that second dose. >> reporter: he says it's critical to get fully vaccinated. the pfizer vaccine was 88% effective against the delta variant after the second dose. but only 33% effective after just one dose. tonight, pfizer launching its phase two and three trial in younger children ages 5 to 11. today at stanford, 6-year-old sophia chavez and her 9-year-old brother nicholas getting the first of two shots, a lower dose of vaccine that what's given to adults. >> so far what we're seeing is the vaccine is just as safe and well tolerated as what we've seen in adults. >> reporter: in new orleans, 7-year-old russell bright getting his shot, too. >> that's why i've been trying see my friends more., so i can - >> and eva pilgrim back with us