Weird, I did not know that.
The study tracked nearly 5,500 people across England over six flu seasons between 2006 and 2011, including the 2009 H1N1, or swine flu, pandemic. Researchers drew blood samples before and after each season from each participant to check for signs of the infection.
They then contacted each household every week to check for cough, cold, sore throat or flu-like illnesses. Those who reported these symptoms then submitted nasal swabs to test for influenza and other respiratory illnesses.
Roughly 1 in 5 unvaccinated people was infected with the flu virus each winter, the study found, but only a quarter of those people showed any symptoms of the infection. And only 17 percent of those infected were sick enough to see a doctor.