Household transmission of the 2009 pandemic A/H1N1 influenza virus: elevated laboratory‐confirmed secondary attack rates and evidence of asymptomatic infections.
BACKGROUND
Characterizing household transmission of the 2009 pandemic A/H1N1 influenza virus (pH1N1) is critical for the design of effective public health measures to mitigate spread. Our objectives were to estimate the secondary attack rates (SARs), the proportion of asymptomatic infections, and risk factors for pH1N1 transmission within households on the basis of active clinical follow-up and laboratory-confirmed outcomes.


METHODS
We conducted a prospective observational study during the period May-July 2009 (ie, during the first wave of the pH1N1 pandemic) in Quebec City, Canada. We assessed pH1N1 transmission in 42 households (including 43 primary case patients and 119 contacts). Clinical data were prospectively collected during serial household visits. Secondary case patients were identified by clinical criteria and laboratory diagnostic tests, including serological and molecular methods.


RESULTS
We identified 53 laboratory-confirmed secondary case patients with pH1N1 virus infection, for an SAR of 45% (95% confidence interval [CI], 35.6%-53.5%). Thirty-four (81%) of the households had ≥1 confirmed secondary case patient. The mean serial interval between onset of primary and confirmed secondary cases was 3.9 days (median interval, 3 days). Influenza-like illness (fever and cough or sore throat) developed in 29% (95% CI, 20.5%-36.7%) of household contacts. Five (9.4%) of secondary case patients were asymptomatic. Young children (<7 years of age) were at highest risk of developing laboratory-confirmed influenza-like illness. Primary case patients with both diarrhea and vomiting were the most likely to transmit pH1N1.


CONCLUSIONS
Household transmission of pH1N1 may be substantially greater than previously estimated, especially in association with clinical presentations that include gastrointestinal complaints. Approximately 10% of pH1N1 infections acquired in the household may be asymptomatic.
Papenburg
Jesse
J
Infectious Disease Research Center of the Centre Hospitalier Universitaire de Québec‐CHUL and Laval University, Quebec City, Canada.
Baz
Mariana
M
Hamelin
Marie-Ève
MÈ
Rhéaume
Chantal
C
Carbonneau
Julie
J
Ouakki
Manale
M
Rouleau
Isabelle
I
Hardy
Isabelle
I
Skowronski
Danuta
D
Roger
Michel
M
Charest
Hugues
H
De Serres
Gaston
G
Boivin
Guy
G
D000293:Adolescent; D000328:Adult; D000914:Antibodies, Viral / Q000097:blood; D058345:Asymptomatic Infections / Q000453:epidemiology*; D002170:Canada; D002648:Child; D002675:Child, Preschool; D005191:Family Characteristics; D005192:Family Health; D005260:Female; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission* / Q000821:virology; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D012367:RNA, Viral / Q000235:genetics; D012307:Risk Factors; D055815:Young Adult
D000914:Antibodies, Viral; D012367:RNA, Viral