Flu Virus May Often Show No Symptoms

Flu virus led to no symptoms in many people with influenza this season

/ Author:  / Reviewed by: Joseph V. Madia, MD Beth Bolt, RPh

(RxWiki News) When a friend or coworker is sick with the flu, people tend to take precautions to avoid getting sick themselves. But it may not always be obvious that someone is carrying the flu virus.

A new study of influenza in the United Kingdom examined data over several different flu seasons.

The researchers found that only around a quarter of those infected with the flu showed symptoms of being ill.

"Talk to your doctor about an influenza vaccination each year."

According to the authors of this new study, which was led by Andrew C. Hayward, MD, of the Research Department of Infection and Population Health at University College London in the United Kingdom (UK), traditional influenza tracking methods might underestimate the presence and reach of the virus.

Dr. Hayward and team aimed to examine various strains of influenza in a variety of age groups over a number of years in order to get a clearer picture of the flu virus.

These researchers looked at five groups of participants across England during 2006 to 2011. Participants reported on any illness experienced each week, and blood tests for influenza were taken both before flu season and after.

After analyzing the data, the researchers determined that on average, the flu virus infected 18 percent of unvaccinated people in the UK each winter. However, they also found that on average, only 23 percent of confirmed flu infections caused any reported illness. In other words, an estimated 77 percent of flu infections caused no symptoms.

Dr. Hayward and team also found that only 17 percent of people with flu visited a doctor or received medical care for their illness.

These results did not vary much when comparing seasonal strains, like H3N2, and pandemic strains, like the H1N1 swine flu.

The researchers also found that rates of infection were usually highest among children between the ages of 5 to 15 years old, and rates generally decreased as age increased.

"Reported cases of influenza represent the tip of a large clinical and subclinical iceberg that is mainly invisible to routine surveillance systems," wrote Dr. Hayward and team, who suggested that perhaps new ways to assess flu levels are needed.

Further research is needed to confirm these findings and explore whether or not results are similar in other countries.

This study was published March 16 by The Lancet Respiratory Medicine. Several study authors reported ties, including receiving funding or speaking fees, to pharmaceutical companies, including GlaxoSmithKline and Baxter.

Review Date: 
March 14, 2014
Last Updated:
March 17, 2014