(HealthDay News) — For individuals infected with influenza, most cases are asymptomatic, and a minority of those with confirmed disease have medically attended illness, according to a study published online March 17 in The Lancet Respiratory Medicine.

Andrew C. Hayward, MD, from University College London, and colleagues compared the community burden and severity of seasonal and pandemic influenza across different age groups and study years. The course of seasonal and pandemic influenza were tracked over five successive cohorts (England 2006–2011; 5,448 person-seasons’ follow-up).

The researchers found that, each winter, influenza infected 18% of unvaccinated people on average. There were 69 respiratory illnesses per 100 person-influenza seasons among those infected with influenza, compared with 44 per 100 in those not infected. Among infected individuals, the age-adjusted attributable rate of illness was 23 illnesses per 100 person-seasons, indicating that most influenza infections are asymptomatic. Of those with serologically confirmed illness, 25% had disease confirmed on polymerase chain reaction (PCR). Among those with PCR-confirmed disease, 17% had medically attended illness. These figures were similar for pandemic and seasonal influenza. Less severe symptoms were seen for those infected with PCR-confirmed 2009 pandemic strain compared with seasonal H3N2.

“Seasonal influenza and the 2009 pandemic strain were characterized by similar high rates of mainly asymptomatic infection with most symptomatic cases self-managing without medical consultation,” the authors write.

Several authors disclosed financial ties to the pharmaceutical industry.

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