The study looked at 216 two- to six-person households in which one family member came down with a confirmed case of 2009 H1N1 swine flu.
Only 13% of family members caught the flu from the infected person. Transmission rates ranged from 28% in two-person families to just 9% in six-person families.
Strikingly, children aged 18 or younger were twice as likely to catch H1N1 swine flu from an infected family member as were family members aged 19 to 50. And people over 50 were about 80% less likely to catch the flu as were younger adults.
"Our results underscore the critical role children play in the unfolding pandemic," note study researchers Simon Cauchemez, PhD, and colleagues.
Even so, children were just as likely as adults to transmit the flu to other family members.
The study also showed that when a family member did catch H1N1 swine flu, symptoms appeared two to four days after symptoms appeared in the first family member that had the flu. On average, the second person in the household to get the flu came down with symptoms 2.6 days after the first person came down with symptoms.
The study is a collaboration between researchers at Imperial College, London, and the CDC. The researchers restricted their study to families of usual size -- two to six members.
People with an illness that met the case definition for H1N1 swine flu were contacted by state and local health departments and interviewed by telephone. Data was then collected on all other household members, defined as anyone who spent at least one night in the house with the infected person in the week after that person came down with flu symptoms.
The study appears in the Dec. 31 issue of the New England Journal of Medicine.
Also appearing in the same issue is an analysis of an H1N1 swine flu outbreak in a New York City school, in which 35% of students and 10% of school employees reported flu symptoms. Symptoms appeared in 95% of infected people within 2.2 days of contact with an infected person. Within the school, each infected person was estimated to have infected 3.3 other people.