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updated 9/28/2010 6:04:22 PM ET 2010-09-28T22:04:22

If the H1N1 virus is to continue its contagious ways this coming flu season, it will have to adapt to a highly immune population, according to a new study.

Epidemiologists estimate that 183 million Americans — 59 percent of the U.S. population — are already immune to the pandemic H1N1 strain of the influenza virus, also known as swine flu.

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People have become immune because they've been either exposed to the virus or vaccinated, the researchers say. Immune people have built up antibodies to defend against the outside invader.

"Every person who is vaccinated is one fewer the virus can find," said study researcher Dr. David Morens, senior adviser to the director of the National Institute of Allergy and Infectious Diseases.

More likely a whimper than a bang
Though there's no percentage that predicts with certainty how forcefully the flu will strike, epidemiologists consider a 59 percent rate high enough for H1N1 to have a great impact unless the virus mutates, Morens said.

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Such mutation is unlikely though not impossible, researchers say.

"Influenza viruses are mutating all the time," Morens told MyHealthNewsDaily. But this particular H1N1 strain is likely to "do it in small sequential steps that will be nothing dramatic, won't cause a big epidemic and, we hope, won't cause a lot of deaths."

Because the H1N1 strain is being included in this year's flu vaccine, the portion of the population that has immunity is likely to grow, Morens added. The injected vaccine contains a dead form of the H1N1 virus.

Have immunity, will travel
The traditional flu season runs from October through March or April.

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Having immunity doesn't necessarily protect a person from getting the flu. Some people with antibodies may still get it, and some who don't have antibodies never will.

But in general, having a high percentage of the population that is immune serves to protect those who aren't, because the virus is less easily passed on from person to person, Morens said. This type of protection is called herd immunity.

In the upcoming season, H1N1 will behave similarly to the 1968 pandemic virus, which caused few deaths, according to the study.

Nineteen percent of the U.S. population was already immune to the H1N1 virus when it came on the scene in March 2009, Morens said. This was probably due to exposure to the 1918 Spanish flu, the ancestor to the modern-day H1N1 strain, he said.

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Older adults may have had immunity because of vaccinations they received in the 1950s, 1960s and 1970s for similar H1N1 viruses.

About a fifth of the U.S. population is now likely to be immune because of vaccinations received in the past year, and another fifth is immune because they were infected with the flu strain, according to the study.

The pandemic H1N1 virus generated a lot of attention last year because it was an unfamiliar virus — with origins in the 1918 Spanish flu — and people hadn't developed immunity to it. However, the virus proved to be not nearly as deadly as feared – it wasn't even as fatal as the typical seasonal flu, Morens said.

Even though health experts say pandemic H1N1 isn't likely to be a big problem this coming season, they urge people to err on the side of caution and vaccinate everyone older than 6 months.

The findings were published Tuesday in the journal mBio.

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