British researchers who tested people for influenza antibodies before and after each flu season for 5 years found that an average of 18% of them appeared to have contracted a flu infection each season, but only 23% of that group got sick, according to a report in The Lancet Respiratory Medicine.
The scientists also found that further tests confirmed flu in only 25% of those whose serologic tests indicated infection, and most of those with confirmed infections didn’t seek medical care. The findings were much the same during the 2009 pandemic and non-pandemic flu seasons.
An additional finding of the Flu Watch cohort study, which focused on people who did not receive flu vaccination, was that the pandemic H1N1 (pH1N1) virus caused milder illness than seasonal H3N2 infections.
The authors, led by Andrew C. Hayward, MD, of University College London, say flu cases detected by national surveillance systems represent only the tip of a very large iceberg. “Underestimation of the number of community cases leads to overestimates of severity,” they write.
Sources: Abstract Comparative community burden and severity of seasonal and pandemic influenza; results of the Flu Watch cohort study. Lancet Respir Med 2014 Mar 17 (Early online publication).
Center for Infectious Disease Research and Policy, 18th March 2014.