doses of pfizer's covid-19 vaccine worldwide. the move comes amid growing concerns at home about the slowing pace of vaccinations and the threat of covid variants. cnn's amber walker is covering all this for us. >> reporter: vaccine donation on a global scale. president biden set to announce the u.s. has bought and will donate 500 million doses of the pfizer covid-19 vaccine by the first half of 2022. biden is set to make the announcement at the g7 summit on thursday. back in the u.s., experts closely watching another variant. >> we don't want to let happen in the united states what is happening currently in the uk where you have a troublesome variant essentially taking over as the dominant variant. >> reporter: growing concern that the more transmissible delta variant first identified in india can fuel new surges among unvaccinated people in the united states. >> if you have states where