Most people dread the flu, and many work hard to avoid it. However, a new British study finds that the typical person over 30 only gets the illness about twice every decade.
"For adults, we found that influenza infection is actually much less common than some people think," said study senior author Dr. Steven Riley, of Imperial College London.
His team published its findings March 3 in the journal PLoS Biology.
"In childhood and adolescence, [flu is] much more common, possibly because we mix more with other people," Riley said in a journal news release. For adults over 30, "the exact frequency of infection will vary depending on background levels of flu and vaccination," he added.
The study team analyzed blood samples from volunteers in southern China to assess their levels of antibodies against nine different flu strains that circulated there between 1968 and 2009.
"This is the first time anyone has reconstructed a group's history of infection from modern-day blood samples," said Dr. Adam Kucharski, who worked on the study at Imperial College London before moving to the London School of Hygiene & Tropical Medicine.
From the blood test results, the investigators concluded that children get the flu an average of every other year, but that flu infections become less frequent as people move through childhood and early adulthood.
After age 30, people tend to get the flu about two times every 10 years, the research team said.
"There's a lot of debate in the field as to how often people get flu, as opposed to flu-like illness caused by something else," Kucharski noted. Even though people may think they have influenza, "symptoms could sometimes be caused by common cold viruses, such as rhinovirus or coronavirus."
On the other hand, flu can sometimes be milder than many people realize. "Some people might not realize they had flu, but the infection will show up when a blood sample is subsequently tested," Kucharski said.
The researchers also developed a model of how people's immune systems change over a lifetime as they encounter different flu strains. The model adds to evidence from previous studies that the flu strains people are exposed to earlier in life trigger a stronger immune reaction than those encountered at later ages.
The study could help improve understanding of how people's immunity affects flu virus evolution, efforts to predict how the flu virus will change in the future, and the effectiveness of vaccines, according to the study authors.
"What we've done in this study is to analyze how a person's immunity builds up over a lifetime of flu infections," Kucharski said. "This information helps us understand the susceptibility of the population as a whole and how easy it is for new seasonal strains to spread through the population."