He added that the low transmissibility rate might mean that people in close contact with someone with the flu did a better job of limiting contact with the sick person, largely because of the enormous publicity devoted to the pandemic.
"It is possible that people were more likely to send household members who were sick to their rooms," Schooley said, "and that the lower transmissibility was only because people were more worried about getting this strain of flu than others that circulate every year."
The finding that children are at greater risk "supports the continued effort to immunize younger people, despite the fact that the second wave has largely dropped off," according to Dr. Andrew Pavia, of the University of Utah in Salt Lake City.
But he cautioned in an e-mail that the study focused on the spring wave of the pandemic and might not completely reflect what's currently happening.
One implication of the findings may be that the emphasis on hygiene has borne fruit, according to Dr. Tracy Zivin-Tutela, of St. Luke's-Roosevelt Hospital Center in New York city.
"The emphasis in the media of hand washing, covering your sneeze and cough, and staying home while ill has all played a role in the decrease in transmissibility of this and all other viruses," she said in an e-mail. "These lessons show that we can decrease the transmission of H1N1 as well as many other respiratory viruses."
In such household studies, the researchers cautioned, the infected patient studied may be more severely affected than usual, which may mean that transmission estimates aren't representative of all cases.
They also noted that households may be more likely to enter a study if they have more cases, which would tend to increase the attack rate.
Cauchemez and colleagues acknowledged that the secondary cases in this study were not confirmed by testing, so that it's likely that some of the cases were not caused by the H1N1 flu.