Other strains of H1N1 virus have been circulating in the United States for decades, including pre-1957, which explains why older people are resistant to infection, Imperato said. Having already been exposed to H1N1, they developed antibodies to the virus that protected from future infections.
H1N1 also reappeared in the late 1970s, causing significant illness in 1978 through the mid-1980s, Imperato said.
Still, experts said the new report should not be read as reason to take swine flu any less seriously.
Children are still being hit unusually hard by the flu. As of mid-November, the CDC estimates some 1,090 U.S. children have died from H1N1, three to four times as many as during a typical flu season, Finelli said.
"Children are at very high risk of hospitalization, are very vulnerable to this flu and should be vaccinated," Finelli said. "We continue to have deaths every day from it, even though a lot of the disease has abated."
Dr. Marc Siegel, an associate professor of medicine at the NYU Langone Medical Center, also recommends people of all ages get vaccinated.
"When kids die, you pay attention," Siegel said. "H1N1 is well worth vaccinating against. It's still something that should be taken seriously and it's not over yet."
Siegel pointed out one weakness of the study is that flu was not confirmed by blood test, but diagnosed by symptoms. That means that other members of the household could have been infected, but were asymptomatic.
"This study only looks at who gets sick, but it leaves out sub-clinical infection," Siegel noted. "What they could be finding is that it's a mild virus, rather than how easily it's spread."
Americans should continue to practice frequent hand washing with warm soapy water or an alcohol-based hand rub if soap and water aren't available; avoid
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