The age patterns observed in the data cannot be explained solely by a difference between community risks for adults and children

The age patterns observed in the data cannot be explained solely by a difference between community risks for adults and children. == RESULTS == An acute respiratory illness developed in 78 of 600 household contacts (13%). In 156 households (72% of the 216 households), an acute respiratory illness developed in none of the household contacts; in 46 households (21%), illness developed in one contact; and in 14 households (6%), illness developed in more than one contact. The proportion of household contacts in whom acute respiratory illness developed decreased with the size of the household, from 28% in two-member households to 9% in six-member households. Household contacts 18 years of age or younger were twice as susceptible as those 19 to 50 years of age (relative susceptibility, 1.96; Bayesian 95% credible interval, 1.05 to 3.78; P = 0.005), and household contacts older than 50 years of age were less susceptible than those who were 19 to 50 years of age (relative susceptibility, 0.17; 95% credible interval, 0.02 to 0.92; P = 0.03). Infectivity did not vary with age. The mean time between the onset of symptoms in a case patient and the onset of symptoms in the household contacts infected by that patient was 2.6 days (95% credible interval, 2.2 to 3 3.5). == CONCLUSIONS == The transmissibility of the 2009 2009 H1N1 influenza virus in households is lower than that seen in past pandemics. Most transmissions occur soon before or after the onset of symptoms in a case patient. As of June11, 2009,a total of17,855 probable or confirmed cases of 2009 H1N1 virus infection, including 45 deaths, had been reported in the United States.1-3The risk factors for transmission of this emerging virus remain largely uncharacterized, particularly in subgroups such as households. Most people who have 2009 H1N1 influenza are advised to stay home until they have been afebrile for at least 24 hours.4This practice puts other household members, who care for the patient when he or she is most infectious, at risk for infection. But so far, neither this risk nor the potential risk factors for transmission have been evaluated in the case of 2009 H1N1 influenza. In this article, we analyze data that describe patients with probable or confirmed cases of 2009 H1N1 influenza, and their household contacts, in Dynamin inhibitory peptide the United States and characterize the Dynamin inhibitory peptide risk factors for transmission in the household, as well as key transmission characteristics of the virus. The household data that are considered here also provide an insight into the way that susceptibility to infection varies with age. Although 60% of the reported cases of 2009 H1N1 influenza in the United States have involved persons who were 18 years of age or younger,3this age distribution might be partly explained by a potential case-ascertainment bias, since children may be tested more often than adults, or by the importance of school clusters in the early phase of the outbreak (with an expected spread to Tnfrsf1b other age groups at a later stage).3However, household contacts of patients with reported infection are expected to be less affected by such case-ascertainment bias. Household studies are also perhaps the most reliable source of data for estimating the serial interval of Dynamin inhibitory peptide the disease the time between the onset of symptoms in a case patient and the onset of symptoms in the household contacts they infect (see theSupplementary Appendix, available with the full text of this article atNEJM.org). Serial-interval estimates are needed to characterize the likely speed at which an epidemic spreads, to inform recommendations with respect to the period of time that patients should stay home, and to estimate the effect of delays in treatment on transmission. == METHODS == == DATA COLLECTION == We defined a case patient as a person with a body temperature of more than 37.8C (100F) and cough or sore throat who was positive for the 2009 2009 H1N1 virus, as assessed with the use of a reverse-transcriptasepolymerase-chain-reaction (RT-PCR) assay (confirmed case) or who was positive for influenza A virus but negative for human H1 and H3 serotypes, as assessed with the use of RT-PCR (probable case).5The RT-PCR assays used for characterizing cases as confirmed or probable were developed by the Centers for Disease Control and Prevention (CDC) (see theSupplementary Appendix). In the early phase of the epidemic (April 29.