WEDNESDAY, Dec. 30, 2009 (HealthDay News) -- People aged 19 and older show more immunity to H1N1 swine flu than was initially believed, a new study finds.
Researchers from the U.S. Centers for Disease Control and Prevention and Imperial College London tracked the spread of H1N1 influenza in 216 households that included a total of 816 people. In each household, one member of the family had been diagnosed with H1N1 during the first wave of swine flu in spring of 2009.
About 13 percent of other household members, or one in eight, came down with the H1N1 flu. Put another way, in 72 percent of households in which one person had the swine flu, no other family member came down with it. In 21 percent of households, one other person got the flu, while in 6 percent, more than one other family member got the flu, the study found.
Children and teens aged 18 and under were twice as likely as those aged 19 to 50 to contract the flu. Those who were 51 and older were less likely still than those in the middle-aged group to contract the flu, according to the study published in the Dec. 31 issue of the New England Journal of Medicine.
"It's probably because people in the middle age group have a little acquired immunity because of prior exposure to another virus that was similar enough," said study co-author Lyn Finelli, lead for the CDC's Epidemiology and Surveillance H1N1 Response Team.
Researchers also found that H1N1 flu was not as easily spread within households as the prior pandemic flu outbreaks of 1957 and 1968.
"The study is significant because the prevailing view has been that the H1N1 virus was very efficiently transmitted, but based on this study, at least at the level of the household, it was not very efficiently transmitted," said Dr. Pascal James Imperato, dean and distinguished service professor in the School of Public Health at State University of New York Downstate Medical Center, in Brooklyn, N.Y.
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 touching the eyes, nose and mouth; avoid close contact with sick family members unless you are caring for them; cover your nose and mouth with a tissue if you cough or sneeze; and stay home if you are sick, advised Finelli.
The study also found, in contrast to conventional wisdom, children were no more likely to spread the flu to other family members than people in other age groups.
Most transmissions of the flu among family members occur either shortly before or after the first patient shows symptoms of infection, said study author Simon Cauchemez, of Imperial College London. The average time between the first person in the household showing flu symptoms and someone else in the household having symptoms was 2.6 days.
In a related study in the same issue of the journal, Johns Hopkins researchers reported on an outbreak of H1N1 flu in a New York City high school. The scientists found that although the infection spread rapidly and widely, it did not cause severe illness for the most part.
For more on H1N1 swine flu, visit the U.S. Department of Health and Human Services.