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Flu can infect many without causing symptoms

Updated: March 17, 2014 09:13 AM
© iStockphoto.com / Nick Schlax © iStockphoto.com / Nick Schlax

SUNDAY, March 16, 2014 (HealthDay News) -- Think you know who has the flu? Think again: a new study finds that three-quarters of people infected with seasonal flu and swine flu in recent years showed no symptoms.

Researchers analyzed data gathered in England during the winter flu seasons between 2006 and 2011, including the 2009 H1N1 "swine flu" pandemic.

Overall, about 18 percent of unvaccinated people became infected with an influenza virus, but only 23 percent of them went on to develop flu symptoms, the researchers reported March 16 in The Lancet Respiratory Medicine.

What's more, only about 17 percent of infected people became ill enough to see a doctor, the British study found. And compared with some of the seasonal flu strains, the 2009 swine flu strain actually caused much milder symptoms.

The findings suggest that relying on data about flu-related visits to primary care doctors underestimates the extent of flu infections and illnesses, the researchers said.

Overall, the infection rate for the winter flu seasons as calculated in the study were an average of 22 times higher than the rates recorded by standard methods.

"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," study lead author Dr. Andrew Hayward, of University College London, said in a journal news release.

"Most people don't go to the doctor when they have flu," he added. "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."

An important question that needs to be answered is whether people who have the flu but have only mild or no symptoms can still easily pass on the virus, Dr. Peter William Horby from the Oxford University Clinical Research Unit in Vietnam, wrote in an accompanying editorial.

Even if only mildly infectious, a large number of these people could play a major role in spreading flu each season, he suggested.

More information

The U.S. National Institute of Allergy and Infectious Diseases has more about influenza.

Copyright © 2014 HealthDay. All rights reserved.

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