Swine flu no longer represents a major threat to the U.S. population, because most people are immune to the virus that caused last season's pandemic, health officials report Tuesday.

Of the 310 million people in the USA, 59% are now believed to be immune to pandemic H1N1 flu, the researchers say. Approximately 62 million people were vaccinated against the virus, 61 million people were infected by it, and another 60 million people 57 or older carry protective antibodies against similar viruses that date back to previous pandemics.

"It's very unlikely that the virus will explode in the fall," says Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), an author of the analysis. "We now have evidence of that."

The evidence comes from studies on the 2009-2010 pandemic carried out by the Centers for Disease Control and Prevention. If this virus follows the pattern set by earlier flu bugs, it will either die out completely or continue to circulate in the ever-shrinking pool of people still susceptible to it, the authors report today in the journal mBio.

That doesn't mean the virus has totally lost its punch, says co-author David Morens, a virologist and historian also at NIAID. "Even with the majority of people in the U.S. immune to the virus, that leaves many tens of millions of people still susceptible," he says. "We know the age groups that are most susceptible — people younger than 55, and those with chronic conditions."

For the first time, federal health officials recommend that all people older than six months get vaccinated against flu, unless they have an allergy to eggs or sensitivity to flu vaccine. Five manufacturers are now churning out 150 million doses of a seasonal flu vaccine that includes the pandemic H1N1 virus. And a more potent vaccine is now available for older people, says Norman Edelman, a pulmonologist at the State University of New York at Stony Brook and president of the American Lung Association.

To reinforce the importance of vaccination, Edelman says, his organization has launched its 5th annual "Faces of Influenza" campaign to demonstrate how severe the illness can be.

"Everyone can benefit from a flu shot," he says.