Want to avoid the dreaded flu this year? It makes good sense to get a flu shot, and if you want to make sure it's effective, smile when you get your jab. In a new study, researchers at the University of Nottingham found that a positive mood has a major impact on the effectiveness of the flu — even more so than diet, sleep, and exercise.

Health experts estimate that the flu vaccine is effective in only 17 to 53 percent of cases in older adults compared to 70 to 90 percent effectiveness for younger patients. So the researchers looked at the factors that could influence the effectiveness of the flu vaccine in older patients.

For the study, the team tracked the mood, diet, exercise and sleep patterns three times a day for 138 older adults (ages 65 to 85) over a period of six weeks prior to getting their flu shots. After the participants were vaccinated, researchers measured the amount of influenza antibodies they produced at four and 16 weeks from the date of injection. Study results were published in the journal Brain, Behavior, and Immunity.

Researchers did not find any link between any of the factors affecting one's physical and psychological health and the effectiveness of the flu shot except for one — positive mood. Overall, participants who reported being in a good mood over the six week period prior to vaccination had higher levels of antibodies in their blood after vaccination. The effect was even more pronounced for those who reported being in a good mood on the day of injection — that factor boosted antibody levels by as much as 8 to 14 percent.

Of course, it's difficult to determine causation in a study like this. Did the positive mood affect immune response or were those with strong immune systems more likely to be in a good mood? That part is not clear. It's likely that there are a number of factors that influence the effectiveness of your flu shot. But it's possible that you might just be able to boost its effectiveness by making sure that you have a good attitude on the day you get vaccinated.