The pandemic influenza A(H1N1) virus does not appear to spread among an infected person's household contacts as easily as viruses in past pandemics, according to an analysis of data collected in the United States.
Simon Cauchemez, Ph.D., of Imperial College, London, and colleagues reviewed information on the H1N1 infection in 216 households; in total, the virus was transmitted from 216 index patients to 600 household contacts. The median age of the index patient was 15 years, and each household had two to six members. Data were collected by the Centers for Disease Control and Prevention.
Overall, 78 (13%) of the 600 household contacts developed acute respiratory illness and 60 (10%) developed an influenzalike illness.
In 156 households (72%), no household contacts developed acute respiratory illness. In 46 households (21%), one household contact developed acute respiratory illness, and in 14 households (6%), more than one contact developed acute respiratory illness. These secondary cases were not systematically confirmed as H1N1 illness (N. Engl. J. Med. 2009;361:2619–27).
In the secondary cases of possible H1N1 influenza, household contacts who were aged 18 years and younger were about twice as likely to develop either acute respiratory illness or flulike illness, compared with household contacts aged 19 years and older. The median age of the household contacts was 26 years, but the median age of contacts with acute respiratory illness was 16.5 years and the median age of contacts with flulike illness was 14.5 years.
The average time between the onset of illness in an index patient and the onset of illness in one of his or her household contacts was 2.6 days.
The estimates of transmissibility in households were lower than those seen in previous pandemics, but they were similar to transmissibility data from the early phase of the H1N1 pandemic in Mexico. No specific symptom was associated with increased transmission of illness, and the findings showed no link between increased transmission of illness and the index patient's age, the researchers noted.
The findings were limited by several factors, including a lack of data about antiviral therapy in household contacts.
Dr. Cauchemez has received consulting fees from Sanofi Pasteur. The study was supported in part by grants from several organizations including the Medical Research Council and the Bill and Melinda Gates Foundation.