Prenatal exposure to the 1957 influenza pandemic and non-affective psychosis in The Netherlands

Second-trimester exposure to the 1957 A2 influenza pandemic is a controvers ial risk factor for schizophrenia. Two earlier studies of the Dutch psychia tric registry failed to find an increased risk for exposed subjects, but di agnostic misclassification within the spectrum of non-affective psychoses h as not yet been ruled out as an explanation for the negative findings. Usin g an enlarged data-set we examined not only whether second-trimester exposu re to the epidemic is associated with an increased risk of schizophrenia (I CD:295), but also whether it is associated with an increased risk of parano id states (ICD:297) or other non-organic psychoses (ICD:298). In this retro spective cohort study the risks of the above-mentioned disorders were compa red for those exposed and unexposed to A2 influenza during the second trime ster of fetal life. The risks for the exposed subjects were not significant ly higher than the risks for the unexposed. The power of the study to detect a significant increase in the risk of schi zophrenia was sufficient. If the relative risk of a lifetime hospitalizatio n for schizophrenia for second-trimester exposed subjects (born January-Apr il 1958) is assumed to be 1.3, the power of the study would be 0.97 (alpha= 0.05; one-tailed testing). If the relative risk for subjects born five mont hs after the peak of the epidemic (mid-February to mid-March 1958) is assum ed to be 1.88, as reported for England and Wales, the power of the study wo uld be close to 1.00. This was the largest study of its kind in Europe: 275 subjects were born in the period January-April 1958 and had a lifetime hos pitalization for schizophrenia. This study indicates that there is no relat ion between second-trimester exposure to the 1957 influenza pandemic and ri sk of non-affective psychosis in the Dutch population. It adds to a growing body of work which does not support an association between maternal influe nza and schizophrenia. (C) 1999 Elsevier Science B.V. All rights reserved.