Household transmission of pandemic (H1N1) 2009, San Antonio, Texas, USA, April-May 2009

Abstract

To assess household transmission of pandemic (H1N1) 2009 in San Antonio, Texas, USA, during April 15-May 8, 2009, we investigated 77 households. The index case-patient was defined as the household member with the earliest onset date of symptoms of acute respiratory infection (ARI), influenza-like illness (ILI), or laboratory-confirmed pandemic (H1N1) 2009. Median interval between illness onset in index and secondary case-patients was 4 days (range 1-9 days); the index case-patient was likely to be < or =18 years of age (p = 0.034). The secondary attack rate was 4% for pandemic (H1N1) 2009, 9% for ILI, and 13% for ARI. The secondary attack rate was highest for children <5 years of age (8%-19%) and lowest for adults > or =50 years of age (4%-12%). Early in the outbreak, household transmission primarily occurred from children to other household members and was lower than the transmission rate for seasonal influenza.

Figure

Texas Department of State Health Services Health Service Region 8 (gray shading), Texas, USA.

Figure A1

Interval between onset of illness for index and secondary case-patients (N = 32), Region 8, Texas, April–May 2009. Bars indicate number of case-patients who had acute respiratory infection (red bar sections), influenza-like illness (white bar sections), or laboratory-confirmed pandemic (H1N1) 2009 (black bar sections). Influenza-like illness, fever (measured or subjective), and cough or sore throat; acute respiratory infection, fever or cough or sore throat or rhinitis.