Prenatal Flu Vaccination Not Linked to Autism in Children

Troy Brown, RN

September 02, 2020

Children born to women vaccinated against the pandemic influenza A(H1N1)pdm09 virus during pregnancy are no more likely to develop autism spectrum disorder (ASD) or autistic disorder (AD) than children born to unvaccinated women, a study found.

"In this large population-based cohort study, we found no association between vaccination of mothers against H1N1 influenza during pregnancy and childhood ASD in the offspring," write Jonas F. Ludvigsson, MD, PhD, from the Department of Medical Epidemiology and Biostatistics at the Karolinska Institutet, Stockholm, Sweden, and colleagues. The researchers report their findings in an article published online August 31 in the Annals of Internal Medicine.

To determine whether there was an association between maternal H1N1 influenza vaccination during pregnancy and ASD risk among the offspring, Ludvigsson and colleauges conducted a population-based cohort study using data from the Medical Birth Register, a Web-based vaccination register for seven healthcare regions in Sweden.

The researchers included singleton live births between October 2009 and September 2010, with follow-up through December 2016.

Infants born during the study period "were in utero at some point during the pandemic vaccination campaign," the authors write.

In the cohort, 29,293 infants were not exposed to the vaccine prenatally, and 39,726 infants were exposed, including 13,845 during the first trimester.

During follow-up (mean, 6.7 years), 394 children in the exposed group were diagnosed with ASD, vs 330 in the unexposed group (cumulative incidence, 1.0% vs 1.1%).

The researchers adjusted for potential confounders, including maternal age at delivery, maternal body mass index, infant sex, maternal parity, maternal smoking, maternal country of birth, disposable income, maternal healthcare region, maternal comorbidity, and prenatal study time.

After adjustment for these factors, there was no association between prenatal H1N1 vaccine exposure and ASD (adjusted hazard ratio [aHR], 0.95) or AD (aHR, 0.96).

Standardized cumulative incidence differences between unexposed and exposed offspring were 0.04% for ASD and 0.02% for AD at 6 years of age.

When the researchers limited the analysis to first trimester vaccination, risk estimates were similar for ASD (aHR, 0.92) and AD (aHR, 0.91).

Study limitations include the fact that the researchers had no data on H1N1 influenza infection in the pregnant women. "If both vaccination and influenza (the latter being less common in persons undergoing vaccination) were risk factors for ASD in the offspring, we may have failed to detect an adverse effect of vaccination," the authors explain.

In addition, they note that they can't rule out residual confounding, because women who are more health conscious may have been more likely to receive the vaccine; however, Swedish health authorities encourage all Swedish residents, particularly pregnant women, to be vaccinated, and there is no cost for the service.

The lack of paternal characteristics could also confound results, the authors add.

The study's strengths outweigh its limitations, Anders Hviid, DrMedSci, from Statens Serum Institut and the University of Copenhagen in Copenhagen, Denmark, writes in an accompanying editorial.

"The Swedish study takes advantage of high-quality, Nordic register data with less concern about selection bias and very limited loss to follow-up due to free, state-funded health care and nationwide, linkable health registers," Hviid writes.

He says it is not surprising that the authors found no link between prenatal influenza vaccination and development of ASD or AD during childhood. "We know that autism has a strong genetic component and that no credible science supports the belief that vaccines administered in pregnancy (or in childhood) can cause autism."

He continues, "In the vaccine safety field, however, credible science and biological plausibility are often not why we conduct studies: We conduct studies to ensure that our vaccination programs are not derailed by spurious accusations."

The Swedish Research Council was the primary funding source for the study. Ludvigsson reports a financial relationship with Janssen outside the submitted work. Pasternak reports receiving grants from the Strategic Research Area Epidemiology program at Karolinska Institutet and from the Swedish Research Council during the conduct of the study. The remaining authors have disclosed no relevant financial relationships. Hviid reports grants from the Novo Nordisk Foundation during the conduct of the study.

Ann Intern Med. Published online August 31, 2020. Abstract, Editorial

Troy Brown is an award-winning Medscape contributor with a special interest in infectious diseases, women's health, and pediatrics.

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