The cause or causes of autism are unknown. Several environmental factors during pregnancy, however, have been associated with an increased risk for this disorder. One such factor is maternal rubella infection. Because natural rubella infection during pregnancy has been shown to increase the risk of autism, Zerbo and coworkers at Kaiser Permanente in Northern California examined the risk of autism in children whose mothers had been infected with influenza virus. These investigators also examined whether influenza immunization during pregnancy increased the risk of autism (Zerbo, O, Qian Y, Yoshida C, et al. Association Between Influenza Infection and Vaccination During Pregnancy and Risk of Autism Spectrum Disorder. JAMA Pediatr. 2016 Nov 28. [Epub ahead of print] doi:10.1001/jama-pediatrics.2016.3609).
To answer these questions, investigators performed a cohort study of 196,929 children of whom 3,103 had autism spectrum disorder who were born at Kaiser Permanente Northern California between Jan. 1, 2000 and Dec. 31, 2010. They found no statistically significant association between either natural influenza infection or influenza vaccination and the development of autism spectrum disorder.
These findings shouldn’t be surprising. Natural rubella virus infection increases the risk of autism because rubella virus enters the bloodstream, crosses the placenta, and infects the unborn child during development. Rubella vaccine virus, on the other hand, because it is significantly attenuated, does not increase the risk of congenital abnormalities if given during pregnancy.
Influenza virus, as distinct from rubella virus, does not cause viremia. Therefore, the virus does not enter the bloodstream and cannot cross the placental barrier. Influenza vaccination, which consists of inactivated virus, also cannot infect the unborn child. Therefore, the findings of these investigators are consistent with the biological differences between rubella and influenza infection and vaccination.