Abstract
BackgroundUnderstanding transmission dynamics of the pandemic influenza A (H1N1) virus in various exposure settings and determining whether transmissibility differed from seasonal influenza viruses was a priority for decision making on mitigation strategies at the beginning of the pandemic. The objective of this study was to estimate household secondary attack rates for pandemic influenza in a susceptible population where control measures had yet to be implemented.MethodsAll Ontario local health units were invited to participate; seven health units volunteered. For all laboratory-confirmed cases reported between April 24 and June 18, 2009, participating health units performed contact tracing to detect secondary cases among household contacts. In total, 87 cases and 266 household contacts were included in this study. Secondary cases were defined as any household member with new onset of acute respiratory illness (fever or two or more respiratory symptoms) or influenza-like illness (fever plus one additional respiratory symptom). Attack rates were estimated using both case definitions.ResultsSecondary attack rates were estimated at 10.3% (95% CI 6.8-14.7) for secondary cases with influenza-like illness and 20.2% (95% CI 15.4-25.6) for secondary cases with acute respiratory illness. For both case definitions, attack rates were significantly higher in children under 16 years than adults (25.4% and 42.4% compared to 7.6% and 17.2%). The median time between symptom onset in the primary case and the secondary case was estimated at 3.0 days.ConclusionsSecondary attack rates for pandemic influenza A (H1N1) were comparable to seasonal influenza estimates suggesting similarities in transmission. High secondary attack rates in children provide additional support for increased susceptibility to infection.
Highlights
Understanding transmission dynamics of the pandemic influenza A (H1N1) virus in various exposure settings and determining whether transmissibility differed from seasonal influenza viruses was a priority for decision making on mitigation strategies at the beginning of the pandemic
Two contacts had onset dates which preceded the primary case by 3 days and were reclassified as primary cases, leaving 87 laboratory-confirmed cases of pandemic (H1N1) 2009 (pH1N1) and 266 household contacts included in this analysis
This study reports on household follow-up of 87 laboratory-confirmed pH1N1 cases in Ontario from late April to early June
Summary
Understanding transmission dynamics of the pandemic influenza A (H1N1) virus in various exposure settings and determining whether transmissibility differed from seasonal influenza viruses was a priority for decision making on mitigation strategies at the beginning of the pandemic. Understanding the transmission dynamics of this novel virus was quickly deemed a priority in order to develop effective mitigation strategies which would minimize transmission until a vaccine was available. Recommended control measures for seasonal influenza epidemics include influenza vaccination, hand hygiene and cough/sneeze etiquette, environmental cleaning and self-isolation; pandemic plans consider travel restrictions, mass immunization, school closures and restriction of mass public gatherings as potentially effective mitigation strategies [2]. While factors influencing pH1N1 transmission were largely unknown at the beginning of the pandemic, seasonal influenza transmission studies had demonstrated the importance of household settings and young children in disease propagation. The role of children in influenza transmission has been demonstrated by Loeb et al (2010) who found that immunizing approximately 80% of children with trivalent influenza vaccine conferred 61% indirect protection for unimmunized residents of Hutterite communities in Western Canada [5]
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