Swine Flu Not As Infectious Among Young Adults As First Feared
Dec. 31 -- WEDNESDAY, Dec. 30 (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.
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