HealthDay News — For individuals infected with influenza as many as three-quarters of cases are asymptomatic, according to a study published in The Lancet Respiratory Medicine, a finding that has implications for disease prevention.

In a population-based comparison of serologic and weekly symptom monitoring data, only 23% of people not vaccinated against influenza who had serologic evidence of infection also reported respiratory or flu-like symptoms, Andrew C. Hayward, MD, from University College London, and colleagues reported.

Furthermore, only 17% of symptomatic influenza cases confirmed with a nasal swab saw their doctor for the flu, the researchers found.

“Reported cases of influenza represent the tip of a large clinical and subclinical iceberg that is mainly invisible to routine surveillance systems,” they wrote.

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Hayward 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 to 2011; 5,448 person-seasons’ follow-up).

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 influenza 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 researchers wrote.

The findings enforce the recommendation that all eligible people be vaccinated against influenza each year, according to the researchers, and have implications for emerging strains of the disease.

“Measurement of the proportion of serologically-confirmed infections that are asymptomatic should be an early priority for any emerging infection of pandemic potential,” they recommended. “This provides an additional index of severity complementing population level data on admission to hospital and deaths.”

In an accompanying editorial, Peter William Hornby, said the study raises questions about the extent to which asymptomatic or mild cases of influenza contribute to disease transmission.

“A large number of well individuals mixing widely in the community might, even if only mildly infectious, make a substantial contribution to onward transmission. This might have important implications for the effectiveness of case isolation and social distancing measures in reducing overall transmission rates,” Hornby wrote.