Flu season in the United States and Europe typically lasts from October through March but outbreaks sometimes linger well into May. Many people who think they have the flu seek medical assistance but a new study suggests that as many as 75% of people infected with the virus don't experience any symptoms at all; they have no idea they're sick and contagious. This extremely high rate of asymptomatic flu patients makes vaccination all the more important for people in high-risk groups, such as during pregnancy.

The study, headquartered at University College in London and led by Dr. Andrew Hayward, followed a select group of families throughout England from 2006 through 2011. This study period included the 2009 H1N1 influenza pandemic.

Approximately 900 volunteers representing 5448 person-seasons were involved in the study. The study group included all household members age 5 and older of families recruited for their demographic representation of the English population as a whole.

The study methodology involved collecting blood samples from each participant before and after flu season each winter. Members of the study research team contacted each household weekly throughout flu season to see if anyone was experiencing cold or flu-like symptoms, coughing, or sore throats. When these symptoms were apparent, the participants provided the researchers with a nasal swab taken the second day after symptoms first appeared. When a study participant sought medical care, the attending physician supplied medical records to the team.

Some findings of the study include:


  • 20% of the general population were infected with seasonal flu and pandemic flu during the study period.
  • 77% did not exhibit symptoms although they were infected.
  • 23% of the infected population exhibited symptoms of the flu.
  • 17% felt ill enough to consult a physician for confirmation of the flu.
  • 18% of the people not vaccinated became infected each year.
  • Symptoms of the H1N1 flu pandemic were milder than flu symptoms in other years.

Hayward says the study indicates most people do not seek medical attention when they have the flu and, when they do, their symptoms are not always identified as influenza. He says the study reveals the number of influenza cases in a given community is greatly underestimated.

This underestimation exaggerates the proportion of flu patients who require hospitalization or die as a result of the flu, according to Hayward. These findings are "critical to inform future control and prevention programs," including emphasizing to pregnant women the need for flu vaccination even if no one in their social circles seems to be suffering symptoms of the flu.

Source: Hayward, Andrew C, MD, et al. "Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study." The Lancet Respiratory Medicine. Elsevier Limited. Mar 17, 2014. Web. Apr 1, 2014.