RELATIONSHIP BETWEEN IN-UTERO EXPOSURE TO INFLUENZA EPIDEMICS AND RISK OF SCHIZOPHRENIA IN DENMARK

Several recent epidemiological studies suggest that exposure to influe nza during gestation increases the risk of later developing schizophre nia, Inconsistency exists, however, particularly in studies that have examined the relationship between the prevalence of influenza and the monthly number of schizophrenic births, over many years, Our sample (N = 9462) was obtained from a Danish computerized case register, and co nsisted of schizophrenic patients born between 1915 and 1970, and firs t admitted to Danish psychiatric hospitals between 1971 and 1991, The study sample was chosen to represent ''incidence cases'' to allow us t o calculate the population attributable risk fraction (PAF). The tempo ral correlation of fluctuations in the prevalence of influenza and flu ctuations in the monthly number of preschizophrenic births was examine d using a Poisson regression analysis, Exposure to influenza 4 months prior to birth (i.e., about the 6th month of gestation) was significan tly associated with an increased risk of later schizophrenia, especial ly for narrowly defined schizophrenia, The number of schizophrenic bir ths was found to have risen by 12% (95% confidence interval: 1-24%) fo r every 100,000 cases of influenza in the 4th month before birth. Our model indicates the PAF to be 1.4%, that is, only 1.4% of the whole sc hizophrenic sample is attributed to prenatal exposure to influenza. Al though maternal exposure to influenza during midgestation is not a maj or risk factor for schizophrenia, the elucidation of its causal mechan ism may open the avenue to understanding the neurodevelopmental origin s of the disease. (C) 1996 Society of Biological Psychiatry.