During flu season, sufferers may marvel at those individuals who just never seem to get sick. But a new study suggests they may actually be ill without knowing it, as three quarters of people with seasonal and pandemic flu do not exhibit symptoms.

The researchers, led by Dr. Andrew Hayward of University College London in the UK, published the results of their study in The Lancet Respiratory Medicine.

They say about 1 in 5 people from the general population were infected in recent outbreaks of seasonal flu as well as the 2009 H1N1 influenza pandemic. However, only 23% of these infections actually caused symptoms.

Additionally, only 17% of people were ill enough to visit a physician.

“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,” Dr. Hayward says.

To investigate further into incidence of flu versus symptoms, he and his colleagues used data from The Flu Watch study, which tracked five consecutive cohorts of households in England during six flu seasons between 2006 and 2011.

Participants of the study provided blood samples before and after each season, and researchers contacted all households weekly to classify any cases of cold, cough, sore throat or “flu-like illness.” In the presence of any such symptom, participants were asked to submit a nasal swab on day 2 of the illness.

The researchers used data from The Flu Watch study to calculate nationally representative estimates of flu incidence, the proportion of infections that were symptomatic, and how many symptomatic infections led to medical attention.

Results of the study reveal that around 18% of the unvaccinated community were infected with flu each winter season and during the 2009 pandemic.

However, 77% of these infections did not show any symptoms, and only 17% of people with confirmed cases of flu visited a doctor.

Additionally, the study showed that compared with some seasonal flu strains, the pandemic strain from 2009 caused much milder symptoms.

Commenting on the findings, Dr. Hayward says:

Most people don’t go to the doctor when they have flu. Even when they do consult, they are often not recognized as having influenza. Surveillance based on patients who consult greatly underestimates the number of community cases, which in turn can lead to overestimates of the proportion of cases who end up in hospital or die.”

As such, he says information on this “community burden is therefore critical to inform future control and prevention programs.”

The team says their findings show that rate of influenza across all winter seasons was around 22 times higher than rates of disease recorded by the Royal College of General Practitioners Sentinal Influenza-Like Illness Surveillance Scheme.

This shows that surveillance in the community has underestimated the magnitude of infection and illness.

Dr. Hayward says we now “need to prepare for how to respond to both mild and severe pandemics,” which can be achieved by refining assessments of severity so that we can react earlier in the face of a pandemic.

Medical News Today recently reported on a study that detailed the discovery of an immune signature, which could predict whether patients newly diagnosed with flu will develop severe symptoms.