>> reporter: mikael dolsten is pfizer's chief scientific officer. do you think there's any chance we might need a fourth shot at some point because of omicron? >> i think it is very likely that we will need a fourth boost, possibly already this spring, particularly if omicron continues to dominate. >> reporter: the head of the c.d.c. said tonight most u.s. omicron cases are in vaccinated people. nearly all have mild symptoms with cough, congestion, and fatigue. meanwhile, the chief scientific officer of pfizer told me they expect to have a vaccine for children as young as six months by the summer. norah. >> o'donnell: that is some news. nikki battiste, thank you. in south africa, where omicron was first discovered last month, cases have jumped from about 200 per day to about 20,000. cbs' debora patta reports >> reporr: firmly he grf a covid storm driven by omicron, south africa's fourth