A new study in PLoS Biology finds that the average adult over age 30 contract the flu just twice every 10 years, a rate much lower than children or adolescents.

For the study, researchers from the Imperial College of London and institutes in the United States and China analyzed blood samples of 151 volunteers in Southern China who were between ages seven and 81. Specifically, they looked at participants' antibody levels against nine different flu strains that circulated between 1968 and 2009.

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In addition to calculating the flu's frequency, the team created a mathematical model of how an individual's immunity to the flu shifts over his or her lifetime as different strains become prevalent.

According to the study, adults typically get the flu twice per decade, while children tend to contract the virus every other year. The study also gave support to the theory that exposure to flu strains earlier in life triggers a stronger immune response than exposure at later ages.

Steven Riley of the Imperial College says, "For adults, we found that influenza infection is actually much less common than some people think," adding, "In childhood and adolescence, it's much more common, possibly because we mix more with other people." However, the study also points out that the frequency of infection also depends on the background on the individual and "levels of flu and vaccination."

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According to Reuters, the data suggest that many other "flu-like illnesses" like coughs and colds are caused by non-influenza bugs, like rhinoviruses and coronaviruses.

The study could be used to enhance understanding of flu virus mutations and the effectiveness of vaccines. More broadly, study co-author Adam Kucharski says, the findings can help "us understand the susceptibility of the population as a whole and how easy it is for new seasonal strains to spread through the population" (Preidt, HealthDay/Philadelphia Inquirer, 3/3; Roberts, BBC News, 3/3; Kelland, Reuters, 3/4).