Estimated three quarters of people with influenza are asymptomatic [Flu Watch study]

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1. Although around 1 in 5 of the unvaccinated population become infected with influenza each winter season, only 23% of these infections were symptomatic.

2. Of infected patients consulting with influenza-like illness, only 8% had influenza or influenza-like illness noted in their medical records.

Evidence Rating Level: 2 (Good)   

Study Rundown: Reliable information on the burden and epidemiology of influenza is critical to disease control and pandemic planning. In this study, Hayward and colleagues assessed the severity and community burden of seasonal and pandemic influenza in England. The study showed that even though less than a quarter of all influenza infections were symptomatic, there were greater proportions of cases identified at times of increased national concern, such as during the 2009 H1N1 pandemic, possibly due to increased propensity to consult and therefore be reported. Overall, the pandemic flu of 2009 was found to be milder in terms of symptom severity than average seasonal influenza. Although the acceptance rate of this study was low with roughly 10% of the invited households participating, the Flu Watch study has strength in its geographic breadth with over 5000 person-seasons recorded over 5 years, providing nationally representative estimates for England.

The study was funded by the Medical Research Council and the Wellcome Trust.

Click to read the study, published today in The Lancet

Relevant Reading: Detection of excess influenza severity: associating respiratory hospitalization and mortality data with reports of influenza-like illness by primary care physicians.

In-Depth [prospective cohort study]: This study recruited 5 cohorts of households across England to measure acute respiratory illnesses and influenza infections during seasonal and pandemic periods. The study recorded 5548 person-seasons during a 5 year follow-up period. Blood samples were taken for serological testing at baseline and at follow up visits each year. Vaccination status and symptoms were self-reported by weekly phone calls or online surveys. Subjects submitted nasal swabs on day 2 of any illness to be screened by RT-PCR for respiratory viruses.

Based on rates per 100 person-seasons, influenza infected on average 18% (95% confidence interval [CI], 16-22) of the unvaccinated population each winter season. PCR-confirmed influenza was identified in 4% (95% CI, 3-5) of the cohort each winter. A minority of those confirmed by PCR met the CDC definition of influenza-like illness (110/238; 46%, 95% CI, 40-53). Most people with PCR-confirmed influenza did not consult. Medical record review among those who did consult showed that only 17% (95% CI, 10-26) with PCR-confirmed influenza and 21% (95% CI, 17-25) with influenza-like illness consulted their family doctors. Finally, among patients consulting with influenza or influenza-like illness, only 8% (95% CI 4-16) had influenza or influenza-like illness recorded in their medical records.

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