The full impact of this season's influenza is yet to be felt, particularly in the East, federal health officials said yesterday... 'We are probably in for a fair amount of activity yet to come over the next weeks,' Dr. Stephen M. Ostroff, a senior epidemiologist at the Centers for Disease Control and Prevention in Atlanta, said in a telephone news conference...He did not say, however, that this would necessarily be a severe season, and Health and Human Services Secretary Tommy G. Thompson said in the same news conference that 'we are hoping that we have got the worst behind us because it started early.'"