Mood disorders and influenza epidemics in Japan

The finding that influenza epidemics are associated with an increased risk of adult mood disorder has been controversial. In this study, data were collected from governmental statistics, the Patient Survey in Japan, in 1996. Index years were defined as 1957/58, 1962, and 1965, and comparison years were defined as 2 years before and 2 years after the index year. The subjects were 361 patients with mood disorders who were born in the index years of influenza epidemics. In order to isolate patients exposed to an influenza epidemic during their second prenatal trimester, the months of birth 5 months after the influenza epidemics were defined as risk exposure months. The proportions of patients born during the exposure period in the index years were compared with those of patients born in the corresponding months in the comparison years. The same procedures were conducted for the periods 2 and 8 months after the epidemics. For female inpatients only, the proportion of patients born in the A2 influenza-exposure months in 1965 with second-trimester exposure was smaller than that of patients born in the corresponding months in the comparison years; the same phenomenon was observed for third-trimester exposure in 1957-58. However, no differences were observed in other comparisons. These findings suggest that prenatal exposure to influenza might decrease the risk for adult mood disorders in females. However, whether there is a causal relationship is still unproven.