PORT WASHINGTON, N.Y. -- Some 72% of Americans polled by the NPD Group said they were concerned about mad cow disease after U.S. officials announced the first case recorded within the country on Dec. 23. The firm's twice-monthly tracking poll had reported only a 57% concern rate at the beginning of December, prior to the disclosure. Nevertheless, the latest poll also found nearly three out of four U.S. consumers said they are confident the foods they buy from supermarkets are safe. That's up six percentage points from the previous survey. Too, despite the concern, 74% of Americans said they intend to eat the same amount of hamburgers or more in the next 30 days.