Don’t worry, be happy — it might make your flu shot more effective.

A new study suggests that older people who are in a good mood when they get the shot have a better immune response.

British researchers followed 138 people ages 65 to 85 who got the 2014-15 vaccine. Using well-validated tests in the weeks before and after their shots, the scientists recorded mood, stress, negative thoughts, sleep patterns, diet and other measures of psychological and physical health. They assessed their antibody response to the vaccine with blood tests at four and 16 weeks after their injections. The study is in Brain, Behavior, and Immunity.

Greater levels of positive mood were associated with higher blood levels of antibodies to H1N1, a potentially dangerous flu strain, at both four and 16 weeks post-vaccination. No other factors measured were associated with improved immune response.

The authors acknowledge they were not able to control for all possible variables, and that their observational study does not prove cause and effect.

The senior author, Kavita Vedhara, professor of health psychology at the University of Nottingham, said that many things could affect vaccine effectiveness, but most are not under a person’s control — age, coexisting illness or vaccine history, for example.

“It’s not there aren’t other influences,” she said, “but it looks like how you’re feeling on the day you’re vaccinated may be among the more important.”