Historically, it's been challenging to pinpoint how often people get the flu, in part because most of us can't self diagnose it, thinking we have it when we don't or not knowing we have it when we do. Now researchers say they've found the answer—that kids get the flu on average every two years, adults every five—by studying antibodies in the blood samples of 151 people in southern China to sort out which influenza strains people have become immune to and when. By comparing a person's antibodies against certain strains with the dates those strains circulated, the researchers were able to ascertain when people actually had the flu, the team reports in the journal PLoS Biology.
"The exact frequency of infection will vary depending on background levels of flu and vaccination," one researcher tells LiveScience. The researchers, from China, the US, and the UK, say there are several possible reasons why kids get the flu more than twice as frequently as adults, among them being that they are exposed to more germs due to increased interaction with others, and also that they've simply not lived as long and haven't accumulated the same antibodies against influenza strains. Interestingly, though, kids have stronger immune responses to the flu than adults. The findings could help researchers better predict how strains will adapt in the future, as well as how immunity to past strains may influence one's response to new vaccines. (This year's flu vaccine was one of the least effective in the past decade.)