Is the U.S. swine flu epidemic over? Federal health officials won't go so far as to say that, but on Friday they reported for the fourth week in a row that no states had widespread flu activity.

U.S. cases have been declining since October. An official with the Centers for Disease Control and Prevention says swine flu cases are still occurring and are likely to continue a while longer at some level.

But another expert said a future large wave of cases now seems very unlikely. The expert, Vanderbilt University's Dr. William Schaffner, said the epidemic has "one foot in the grave."

An estimated 70 million Americans have been vaccinated against swine flu through a government vaccination campaign that started in October. Meanwhile, an estimated 55 million or more got sick from swine flu and recovered, meaning they developed some level of natural immunity from the experience.

Combined, that means that 40 percent or more of the public has immunity to the virus. Such numbers will make it difficult for swine flu in its current form to reappear in a new wave, Schaffner noted.