London study finds 77 percent of influenza infections are asymptomatic
The study, titled "Flu Watch" and published in The Lancet Respiratory Medicine journal, observed five groups of households in England over the course of six influenza seasons, from 2006 to 2011. The researchers found that during recent seasonal outbreaks and the 2009 H1N1 pandemic, roughly 20 percent of the study population was infected with the influenza virus, but only 23 percent of cases were symptomatic and only 17 percent of persons with symptoms sought medical attention.
"Reported cases of influenza represent the tip of a large clinical and subclinical iceberg that is mainly invisible to national surveillance systems that only record cases seeking medical attention," Lead Study Author Dr. Andrew Hayward said.
The researchers said influenza surveillance is based on patients who seek medical attention, but community cases are often overlooked. They said that whether or not a person is symptomatic, they are still contagious and more should be done to control infection.
"Surveillance of medically attended illnesses provides a partial and biased picture, and is vulnerable to changes in consulting, testing, or reporting practices," Dr. Peter William Horby of Oxford University's Clinical Research Unit in Vietnam said. "As such, it is clear that reliable estimates of the infection and clinical attack rates during the early stages of an influenza epidemic requires the collection of standardized data across the whole range of disease severity, from the community, primary care, and secondary care."