Attack rates assessment of the 2009 pandemic H1N1 influenza A in children and their contacts: a systematic review and meta-analysis

Abstract

Background: The recent H1N1 influenza A pandemic was marked by multiple reports of illness and hospitalization in children, suggesting that children may have played a major role in the propagation of the virus. A comprehensive detailed analysis of the attack rates among children as compared with their contacts in various settings is of great importance for understanding their unique role in influenza pandemics.

Methodology/principal findings: We searched MEDLINE (PubMed) and Embase for published studies reporting outbreak investigations with direct measurements of attack rates of the 2009 pandemic H1N1 influenza A among children, and quantified how these compare with those of their contacts. We identified 50 articles suitable for review, which reported school, household, travel and social events. The selected reports and our meta-analysis indicated that children had significantly higher attack rates as compared to adults, and that this phenomenon was observed for both virologically confirmed and clinical cases, in various settings and locations around the world. The review also provided insight into some characteristics of transmission between children and their contacts in the various settings.

Conclusion/significance: The consistently higher attack rates of the 2009 pandemic H1N1 influenza A among children, as compared to adults, as well as the magnitude of the difference is important for understanding the contribution of children to disease burden, for implementation of mitigation strategies directed towards children, as well as more precise mathematical modeling and simulation of future influenza pandemics.

Figure 1. Flow chart for study selection.

Figure 2. Relative risk of attack rates of the 2009 pandemic H1N1 influenza A in children and adults during school outbreaks.

Graphic representation of laboratory confirmed attack rates relative risk (95% CI) in children versus adults in school outbreaks. (Top panel) Laboratory-confirmed attack rates. (Bottom panel) Clinical attack rates.

Figure 3. Relative risk of secondary attack rates of the 2009 pandemic H1N1 influenza A in children and adults for household outbreak investigations.

Graphic representation of secondary attack rates relative risk (95% CI) in children versus adults. (Top panel) Laboratory-confirmed secondary attack rates for laboratory confirmed contacts of laboratory confirmed index cases. (Bottom panel) Secondary attack rates for clinically diagnosed contacts (with ILI) of laboratory confirmed index cases. In studies marked with an asterisk (*), all contacts were laboratory tested for the presence of the 2009 pandemic H1N1 influenza A virus.