Estimated Effectiveness of Influenza Vaccines in Preventing Secondary Infections in Households

Households, where individuals often interact in close contact, provide a favorable environment for transmission of respiratory viruses. Previous household studies indicated that 3% to 38% of household members would be infected after a first member was infected with influenza and identified school-aged children as important contributors to influenza transmission in households.1-7 Yet, most of those seminal studies were conducted several years ago or during the 2009 pandemic, and more recent prospective assessments of transmission of influenza in US households are scarce.

Influenza vaccines are recommended in the US for individuals aged 6 months and older for the prevention of influenza and related complications. While the effectiveness of influenza vaccines in preventing medically attended illness and serious disease leading to hospitalization has been demonstrated and is monitored closely,8-12 the effectiveness of influenza vaccines in preventing infections regardless of symptoms remains unclear and is uncommonly studied.13 The degree to which vaccines reduce secondary infections after exposure to influenza, especially in high transmission settings, like households, may be especially informative to individuals at increased risk for severe influenza complications.14

In collaboration with the US Centers for Disease Control and Prevention (CDC), we conducted the Influenza Transmission Evaluation Study (FLUTES), a case-ascertained household transmission study at 2 research sites during 3 consecutive influenza seasons in the US. We assessed the incidence of laboratory-confirmed influenza virus infection in household members with the specific objective of estimating secondary infection risks and the effectiveness of influenza vaccines in preventing secondary infections in household contacts.