Study raises questions about age-group impact of H1N1

French researchers now believe that the effects of H1N1 may not have differed from seasonal flu epidemic effects on certain age groups as much as was initially believed.

Magall Lamaitre and Fabrice Carrat, the authors of a report published by BMC Infectious Diseases, said that the initial general understanding of H1N1 is that it affected children and young adults while generally leaving elderly unscathed.

The authors say the new study seems to show that younger people had a greater share of H1N1 deaths than they have from seasonal flu. The age distribution of total cases, however, is almost the same as in past seasonal flu epidemics.

“The age distribution of influenza-like illness was similar between the 2009 H1N1 pandemic and seasonal epidemics whereas the proportion of under-60s among influenza deaths was markedly higher during the 2009 pandemic - peak<20 years - than during the seasonal epidemics,” the authors wrote.

Cecile Viboud, PhD, a staff scientist in the division of international epidemiology and population studies at the National Institutes of Health's Fogarty International Center, said she was surprised by some of the study’s findings, but said the study may not be without some flaws.

She noted that it was possible that the study lacked the statistical power to detect changes in the age distribution of cases in pandemics.

“This is mostly because they only have two seasonal epidemics to compare with, and also because the morbidity data come from several different studies with likely large variations in sampling and testing practices,” Viboud told CIDRAP News.

Viboud said another potential flaw in the study is its use of data on deaths specifically listed as influenza, which could underestimate the true flu burden.