setbacks in early april due to safety concerns about blood clots in a very small number of women, saying today it is working to try to extend the shelf life of its doses. >> we still believe that we are going to be very relevant in terms of getting patients vaccinated around the world. >> reporter: this coming amid president biden's announcement that the u.s. will buy 500 million doses of pfizer to provide to poorer nations. the race to vaccinate in the u.s. facing challenges even as concerns grow about the highly contagious delta variant first detected in india. that strain now dominant in the uk, with transmission rising among 12 to 20-year-olds, raising concerns for experts here in the u.s. >> we're out there really pushing and beating the bushes and trying to get people to understand why it's so important for them to get vaccinated. >> reporter: on average, about 357,000 u.s. adults have gotten at least one shot every day in the last week, down 19.7%, far short of the approximately 540,000 needed daily to hit