Many people who skip seasonal influenza shots contract the disease unknowingly, often mistaking milder symptoms for the common cold.

A new study led by epidemiologist Andrew Hayward at University College London finds inflluenza more common than previously thought. In fact, three-quarters of people who get the flu don’t realize they’re carrying the disease and may be contagious.

"[The] flu is more common than we thought, but often less severe than what we had thought," Hayward told NPR. "A lot of the time you may just have a runny nose, a bit of a cough, perhaps a sore throat.”

On average, 18 percent of people get the flu every winter, Hayward and his colleagues found after reviewing data for seasonal and pandemic influenza strains from 2006 to 2011 in the United Kingdom. They tracked nearly 5,500 people during those six years, taking blood samples at the beginning and end of seasonal flu season.

In a paper published Monday in The Lancet Respiratory Medicine, Hayward says many people infected with the 2009 pandemic "swine" flu strain developed milder symptoms than those bitten by the seasonal H3N2 bug. Only 17 percent of those who got the flu even felt sick enough to visit a doctor — in a country with universal access to health care, no less.

The findings surprised epidemiologists who’d assumed the 2009 pandemic strain would cause stronger symptoms than the seasonal flu, given the severity of the H1N1 pandemic flu. Hayward says the study shows how influenza presents public health officials with a mixed threat requiring improved understanding of new flu strains.

"If you want to understand how severe it is, you really need to understand not only the number of people who end up in hospitals or die but also the number of people in the community who are infected in the first place," Hayward said. "So we need better methods of picking those people up."

Doctors may need better diagnostic tools to improve epidemiological detection of such flu strains, flying below the radar.

Source: Hayward A, Fragaszy EB, Bermingham A, Wang L, Copas A, Edmunds J. Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study. The Lancet Respiratory Medicine. 2014.