Swine flu no longer represents a major threat in the U.S. because so many people are immune to the virus that caused last season's pandemic, health officials reported Tuesday.
Of the 310 million people in the U.S., 59% are 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 to similar viruses that circulated years ago.
"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 a flurry of studies of the 2009-10 pandemic carried out by the Centers for Disease Control and Prevention, the researchers say. If this virus follows the pattern set by earlier flu bugs, it will die out completely or circulate among people who are still susceptible a year and a half after the virus emerged, the authors report in the journal mBio.
There are still many people who are vulnerable to swine flu, says David Morens, a virologist and historian at NIAID who co-wrote the report with Fauci and colleague Jeffery Taubenberger. "Even with the majority of people in the U.S. immune to the virus, that leaves many tens of millions of people still susceptible," Morens says. "We know the age groups that are most susceptible -- people younger than 55 and those with chronic conditions."
The CDC now estimates the 2009 H1N1 flu killed roughly 12,500 and sent 274,000 people to the hospital with respiratory diseases and other complications, a toll that mirrors the latest averages for people who suffer from seasonal flu. The death rate was relatively low compared with other pandemics, especially the 1918 flu, which killed 675,000 people in the U.S., Morens says.
So far, he says, the virus hasn't developed "some terrible new mutation" that would make it more lethal, though that could still occur.
For the first time, federal health officials recommend that all people older than 6 months get vaccinated against flu, unless they have an allergy to eggs. Five manufacturers are churning out 150 million doses of a seasonal flu vaccine that includes the 2009 H1N1 virus. A more potent one is available for seniors, says Norman Edelman, preventive medicine professor at the State University of New York-Stony Brook and chief medical officer of the American Lung Association.
To reinforce the importance of vaccination, Edelman says his organization began its annual "Faces of Influenza" campaign to show how severe flu can be. "Everyone can benefit from a flu shot," he says.
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