Previous research has tied prenatal influenza exposure to mental retardation, explain Dr. Willy Eriksen and co-authors at the Norwegian Institute of Public Health in Oslo. However, the effect of prenatal exposure to epidemic influenza on mean intelligence in the general population has yet to be explored.
The Honk Kong flu, which had antigenic properties from both an avian virus and a human virus, emerged worldwide in the late 1960s. Norway was affected primarily between November 1969 and January 1970, with an estimated attack rate of 15% to 40%.
To assess the impact of prenatal exposure on intelligence in adulthood, Dr. Eriksenís team linked registry data for nearly 183,000 persons from the Medical Birth Register of Norway, the National Conscript Service, and Statistics Norway.
The analysis focused on males who would have been exposed during the first months of gestation during the epidemic and born in summer and autumn of 1970. IQ test results taken at age 18 were compared to those of persons born in 1967-1969 and 1971-1973.
"The mean intelligence score increased monotonously from one birth year to another, except for a downturn in the birth year 1970,î the authors note, which ìrepresented a highly significant deviation from the linear trend of increasing scores over the 7 birth years."
Furthermore, only in 1970 was the mean intelligence score of men born between July and October lower than scores of those born in other months of the same year.
Dr. Eriksen’s group estimates that the difference corresponds to 3 to 7 points on a standard IQ scale. However, "if cerebral complications occurred in only a small group of those who were exposed," they maintain, "the detrimental effects on the intelligence of the susceptible individuals may have been considerably larger."
Ann Neurol 2009.