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. 2014 Sep 25;9(9):e108485.
doi: 10.1371/journal.pone.0108485. eCollection 2014.

Household transmission of influenza A(H1N1)pdm09 in the pandemic and post-pandemic seasons

Itziar Casado  1 , Iván Martínez-Baz  2 , Rosana Burgui  2 , Fátima Irisarri  2 , Maite Arriazu  2 , Fernando Elía  3 , Ana Navascués  4 , Carmen Ezpeleta  4 , Pablo Aldaz  5 , Jesús Castilla  2 ; Primary Health Care Sentinel Network of Navarra
Collaborators, Affiliations

Household transmission of influenza A(H1N1)pdm09 in the pandemic and post-pandemic seasons

Itziar Casado et al. PLoS One. .

Abstract

Background: The transmission of influenza viruses occurs person to person and is facilitated by contacts within enclosed environments such as households. The aim of this study was to evaluate secondary attack rates and factors associated with household transmission of laboratory-confirmed influenza A(H1N1)pdm09 in the pandemic and post-pandemic seasons.

Methods: During the 2009-2010 and 2010-2011 influenza seasons, 76 sentinel physicians in Navarra, Spain, took nasopharyngeal and pharyngeal swabs from patients diagnosed with influenza-like illness. A trained nurse telephoned households of those patients who were laboratory-confirmed for influenza A(H1N1)pdm09 to ask about the symptoms, risk factors and vaccination status of each household member.

Results: In the 405 households with a patient laboratory-confirmed for influenza A(H1N1)pdm09, 977 susceptible contacts were identified; 16% of them (95% CI 14-19%) presented influenza-like illness and were considered as secondary cases. The secondary attack rate was 14% in 2009-2010 and 19% in the 2010-2011 season (p=0.049), an increase that mainly affected persons with major chronic conditions. In the multivariate logistic regression analysis, the risk of being a secondary case was higher in the 2010-2011 season than in the 2009-2010 season (adjusted odds ratio: 1.72; 95% CI 1.17-2.54), and in children under 5 years, with a decreasing risk in older contacts. Influenza vaccination was associated with lesser incidence of influenza-like illness near to statistical significance (adjusted odds ratio: 0.29; 95% CI 0.08-1.03).

Conclusion: The secondary attack rate in households was higher in the second season than in the first pandemic season. Children had a greater risk of infection. Preventive measures should be maintained in the second pandemic season, especially in high-risk persons.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Flow chart of households of index cases with laboratory-confirmed influenza A(H1N1)pdm2009 and their contacts in the study of the 2009–2010 and 2010–2011 influenza seasons in Navarra, Spain.
See this image and copyright information in PMC

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