Researchers say that of the United States' 310 million people, 59 percent are believed to be immune to the H1N1 flu virus, USA Today reported.
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 flu outbreaks, officials say.
"It's very unlikely that the virus will explode in the fall," Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, says. "We now have evidence of that."
Studies of the 2009-2010 pandemic by the Centers for Disease Control and Prevention suggest the H1N1 virus will either die out completely or continue to circulate in the ever-shrinking pool of people still susceptible to it, researchers say.
That doesn't mean the virus won't be a problem, NIAID virologist David Morens says.
"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.
For the first time, federal health officials are recommending that all people older than six months get vaccinated against flu, USA Today reported.