Abstract
BackgroundThe 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.MethodsDuring 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.ResultsIn 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).ConclusionThe 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.
Highlights
Influenza transmission in human beings occurs primarily via the droplet and contact routes [1]
The World Health Organization (WHO) estimated a secondary household attack rate of the influenza A(H1N1)pdm09 virus of between 22% and 33%, considerably higher than the secondary attack rate for seasonal influenza, which is between 5% and 15% [3]
Symptom onset in the index cases included in the study in the 2009–2010 season occurred between 20 October and 17 November 2009, and in the 2010– 2011 season between 3 January and 25 February 2011
Summary
Influenza transmission in human beings occurs primarily via the droplet and contact routes [1]. Influenza A(H1N1)pdm virus was highly transmissible in schools and households [2]. Several studies had evaluated household transmission of seasonal influenza [4,5,6]. The emergence of a new influenza virus during the 2009–2010 season sparked interest in studying the transmission during the pandemic [7,8,9]. 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)pdm in the pandemic and post-pandemic seasons
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