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Over 75% of flu victims do not have any symptoms

Teresa Tanoos's picture
Study says 75% of people with the flu don't have any symptoms.

New study says more than three-quarters of people with the flu do not have any symptoms.


When someone has the flu, they may suffer symptoms ranging from fever, aches and pains - to congestion, wheezing and coughing. But a new study says that more than 75 percent of people who had seasonal influenza during recent years did not experience any symptoms at all.

Researchers for the study collected data for influenza during winter seasons in England between 2006 and 2011, which included the H1N1 flu pandemic in 2009.

After analyzing the data, the research team found that approximately 18 percent of those who did not get a flu shot became infected with a flu virus. Among them, however, only 23 percent developed any symptoms of the flu.

The findings of the study, published March 16, 2014 in The Lancet Respiratory Medicine, also found that only an approximate 17 percent of people infected with the flu virus went on to develop symptoms severe enough to warrant a visit to the doctor.

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Indeed, victims of the 2009 “swine flu” experienced much milder influenza symptoms, compared with some of the seasonal flu strains during other years. Therefore, basing the number of flu-related cases on doctor visits ends up underestimating how many people actually had influenza during any given year.

This was evident after the researchers for the study calculated flu infection rates between 2006 and 2011. As a result, they found that the overall rate was, on average, 22 times higher than the rates recorded by standard surveillance systems that were based on flu-related doctor visits only.

In a news release, lead study author, Dr. Andrew Hayward of University College London, said that the number of reported cases of the flu only represent the “tip of a large clinical and subclinical iceberg,” which is primarily “invisible to surveillance systems” that only record flu cases that require a doctor’s care or hospitalization.

Dr. Hayward also pointed out that most people with the flu never visit their doctor – and even when they do, they are frequently misdiagnosed with something other than the flu. As a result, standard surveillance methods end up underestimating the actual number of flu cases because they only record how many flu patients visit their doctor or are otherwise hospitalized. This, Hayward added, leads to an overestimation "of the proportion of cases who end up in the hospital or die."

In an editorial that accompanied the report published in The Lancet Respiratory Medicine, Dr. Peter William Horby of Oxford University in Vietnam, noted that it’s important to find out if those who have influenza with no symptoms or only mild ones are contagious. If they are, then even those whose symptoms are only mild could pass on the virus infection easily; thus, spreading influenza to a larger portion of the population each flu season.

SOURCE: Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study, Andrew C Hayward, et al., The Lancet Respiratory Medicine, published online 17 March 2014.