Concerns about potential epidemics of influenza have stimulated interest in acquiring better understanding of its transmission within communities and households. This information could help guide preventive strategies such as targeted immunization and the administering of neuraminidase-inhibiting medications to close contacts of those infected. Viboud and colleagues analyzed a prospective study to determine risk factors for influenza transmission within households during the 1999 to 2000 influenza season in France.

The study used 161 of the family physicians in the French Sentinel network, a computerized public health surveillance system that monitors 10 communicable diseases on a weekly basis. These physicians enrolled 946 index patients and their household contacts in a three-month period. Index patients were included in the study if they presented to a participating physician with respiratory symptoms and fever consistent with the diagnosis of influenza. Nasal swabs were obtained from all index patients, and for the 15 days after consultation, data regarding 13 specific symptoms were collected daily from their household contacts. A secondary patient was defined as a household contact who developed symptoms consistent with clinical influenza within five days of illness in the index patient.

Influenza A was confirmed in 510 index patients; 395 (77 percent) of these completed the household contact follow-up protocol. Clinical influenza was transmitted to 131 household contacts overall. Transmission occurred in 97 (35 percent) of the households: in 67 households (69 percent), there was only one secondary patient, in 26 (27 percent) there were two, and in four households (4 percent) there were three. The median time between onset of symptoms in the index patient and in the secondary patient was two days (ranging from one to five days). A comparison of secondary patients with uninfected household contacts revealed no significant differences in age, sex, smoking status, history of chronic disease, immunization against influenza, or previous influenzal illnesses between the two groups. Transmission of influenza was most likely to occur when the index patient and contacts were children, especially those younger than five years.

The researchers conclude that young children are a major factor in the transmission of influenza within households. They attribute the higher susceptibility of children to a combination of factors. Young children are exposed to a large group of non-household contacts through daycare and preschool activities and hence are more at risk of infection. They also may have lower immunity to infection than older family members, plus increased viral shedding and longer duration of illness. Viboud and colleagues estimate that rapid influenza testing could identify 72 to 95 percent of children with influenza during the initial physician visit. Effective prophylaxis of household contacts, focusing on the youngest, could prevent 21 to 41 percent of secondary cases.