Household transmission of pandemic 2009 influenza A (H1N1) virus in Osaka, Japan in May 2009.
OBJECTIVE
To assess household transmission of pandemic influenza A (H1N1) and effectiveness of postexposure prophylaxis (PEP) of antiviral drugs among household contacts of patients during the first pandemic influenza A (H1N1) outbreak in Osaka, Japan in May 2009.


METHODS
Active surveillance of patients and their families was conducted. Public Health Center staff visited each home with an infected patient and advised every household member with regard to precautionary measures, and PEP was provided to household contacts to prevent secondary infection. We analyzed the effectiveness of PEP and characteristics of secondary infection.


RESULTS
The secondary attack rate (SAR) among household contacts was 3.7%. The SAR among household contacts without PEP was 26.1%. However, the SAR among those with PEP was 0.6%. Only two of 331 household contacts with PEP became infected. One of the two was infected with an oseltamivir-resistant strain. Analysis of SAR by age group showed that those under 20 years of age were at higher risk than those over 20 (relative risk [RR] = 7.9; 95% confidence interval [CI] = 2.24-27.8). Significant differences with respect to sex, number of household contacts, and use of antiviral medications in the index cases were not observed.


CONCLUSIONS
Our present results indicate that PEP is effective for preventing secondary H1N1 infection among household contacts.
Komiya
N
N
Field Epidemiology Training Program, National Institute of Infectious Diseases, 1-23-1 Toyama, Shinjyuku-ku, Tokyo, Japan. kominobu@nih.go.jp
Gu
Y
Y
Kamiya
H
H
Yahata
Y
Y
Yasui
Y
Y
Taniguchi
K
K
Okabe
N
N
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000998:Antiviral Agents / Q000627:therapeutic use*; D018890:Chemoprevention / Q000379:methods*; D002648:Child; D002675:Child, Preschool; D018562:Disease Transmission, Infectious / Q000517:prevention & control*; D005191:Family Characteristics; D005192:Family Health; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D007251:Influenza, Human / Q000635:transmission* / Q000821:virology*; D007564:Japan; D008297:Male; D008875:Middle Aged; D055815:Young Adult
D000998:Antiviral Agents