ATLANTA -- If the U.S. swine flu epidemic isn't over, it certainly looks as if it's on its last legs.
While federal health officials are not ready to declare that the threat has passed and the outbreak has run its course, they did report Friday that for the fourth week in a row, no states had widespread flu activity. U.S. cases have been declining since late October.
One U.S. expert said the epidemic has "one foot in the grave," and there are many reasons to believe that there won't be another wave later in the year.
For one thing, the virus has shown no signs of mutating. The vaccine against it is effective. And roughly half the people in the U.S. probably have some immunity because they were infected with it or got vaccinated.
The World Health Organization is witnessing an international decline as well and is discussing criteria for declaring the pandemic over.
"Clearly, the last four weeks have been one of the quietest January flu seasons I can remember in my career," said Michael Osterholm, a prominent expert on global flu outbreaks with the University of Minnesota.
Since its emergence in April, swine flu has caused an estimated 15,200 deaths worldwide, mostly in the U.S. -- a much lower number than initially feared. The positive outcome is primarily because the virus didn't mutate into a deadlier form.
Even so, experts have praised the actions of the U.S. and Mexican governments and of scientists who quickly developed a vaccine.
Criticizing the government for its intense response would be like chastising officials for building dikes in New Orleans to withstand a Category 5 hurricane and then seeing only a Category 3 come ashore, Osterholm said.
"The government did not overreact," said Dr. Arnold Monto, a University of Michigan flu expert.
Whether it will stay quiet for the rest of the winter is hard to say, but some experts are beginning to lean that way.
"If it's not dead, it's weakening fast. It's got one foot in the grave," said Dr. William Schaffner, a flu authority at Vanderbilt University.
A poll released Friday by the Harvard School of Public Health found that 44 percent of Americans believe that the outbreak is over.
The Centers for Disease Control and Prevention released numbers Friday showing that most states continued to have only occasional flu activity last week. However, only three states had absolutely no reports, and a CDC official cautioned that swine flu is still around.
"We don't seem to be seeing the disappearance of this virus," said Dr. Anne Schuchat.