When people get sick with the flu, their immune systems produce antibodies that target proteins on the virus surface, and after a person recovers, "the infection will show up when a blood sample is subsequently tested," Kucharski said. Upon subsequent infection, it has been suggested that the original response can be enhanced [14,19–21] and the antibody response to the new strain reduced [22–26]; for original antigenic sin to occur, there is evidence that the original and new strain must be antigenically related [26,27]. This is the first time that anyone has reconstructed a group's history of flu infection from modern-day blood samples, Kucharski said. They looked at blood samples from 151 people living in southern China, testing each one against a panel of nine strains of H3N2 that were in circulation between 1968 and 2009, according to the study published today (March 3) in the journal PLOS Biology. In the study, the researchers focused on influenza A (H3N2), a dominant strain of flu virus that sickens and kills the most people worldwide, Kucharski said in a statement. The findings will help scientists understand how a population's immunity influences the flu as the virus evolves, and may help researchers predict how the circulating strains of virus may change in the future, the researchers said. Integrating the life course of immunity into future analyses of influenza dynamics could therefore lead to a better understanding of population susceptibility and the potential transmissibility of new seasonal strains.
– 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.)