A study of household transmission of the H1N1 pandemic flu has confirmed that children 18 and younger are more at risk than young adults and seniors.
Compared with adults ages 19 through 50, children in households where someone had confirmed H1N1 flu were twice as likely to become infected, according to Simon Cauchemez, of Imperial College London and colleagues there and at the CDC.
Adults older than 50, on the other hand, were less likely to come down with the disease, Cauchemez and colleagues reported in the Dec. 31 issue of the New England Journal of Medicine.
The study also showed that the transmission rate of the virus is relatively low, the researchers said.
Since the start of the pandemic in April, children have been seen to be at greater risk, Cauchemez told MedPage Today, but the observations might have been explained by a case-ascertainment bias, since children would be more likely to be tested than adults.
It might also have been the case that school clusters -- which played a role in the early phase of the outbreak -- were biasing observations, he said.
The current study, Cauchemez said, "narrows the assumptions we can make on why there are more children among cases," he said. It could be, he said, that children have weaker immunity to flu, or there might be social interactions that make them more vulnerable.
To help understand the age effects, as well as other factors involved in transmission, the researchers used data collected by the CDC on 938 households in which there was a confirmed or probable case of the H1N1 flu, the so-called index patient.
For this study, the researchers looked at a subset of 216 households in which the index patient reported having household contacts and data was complete for all 600 of them.
Only 78 of the 600 household contacts, or 13 percent, developed an acute respiratory illness.
In nearly three-quarters of the households -- 156 -- no other member fell ill, while one contact fell ill in 46 and two or more developed illness in 14 households.
Household contacts 18 or younger were twice as susceptible as those age 19 to 50.
Those older than 50 were less susceptible.
On average, the time between onset of symptoms in a case patient and onset in a household contact was 2.6 days.
Why The Young May Catch H1N1 More Often
The risk to younger people was reflected in the fact that the median age of all household members was 26, while the median age of those who developed an acute respiratory illness was 16.5, the researchers said.
On the other hand, children and adults with the flu were found to be equally infectious to others, they said.
The 13 percent rate of transmission is at the low end of the scale both for seasonal flu (where it ranges from 10 percent to 40 percent) and for other pandemics, Cauchemez and colleagues said.
That transmission suggests that people who have housemates infected with H1N1 shouldn't consider that they're out of the woods themselves, according to Dr. Robert Schooley, of the University of California San Diego.
"It argues that people who have had a case of presumed H1N1 in their household shouldn't assume that they have had flu and skip being vaccinated," he said in an e-mail.
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.