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Children born to women receiving the 2009-2010 H1N1 influenza vaccine during pregnancy had no higher likelihood of developing autism later in life, a Swedish-based population study found.

Babies exposed to the "swine flu" vaccine during pregnancy did not have an increased risk of developing autism spectrum disorder (ASD) or autistic disorder (AD) compared to those born to unvaccinated women, reported Jonas F. Ludvigsson, MD, PhD, of Karolinska Institutet in Stockholm, and colleagues.

In an analysis of first trimester exposures -- a period when vaccines could potentially have the most profound effect on fetal neurodevelopment -- there was still no link between immunization and autism risk, researchers wrote in .

"Both our main analyses... and several additional analyses all show that H1N1 influenza vaccination during pregnancy is not linked to autism in the offspring," Ludvigsson told MedPage Today in an email.

In , Anders Hviid, MSc, DrMedSci, of the Statens Serum Institut and University of Copenhagen in Denmark, stated that this research "is a rare and welcome contribution to a more comprehensive safety evaluation, which goes significantly beyond the perinatal period."

While the biological plausibility that influenza vaccines may harm fetal development is weak, Hviid emphasized a need for further observational studies that examine the effects of vaccinations not only during pregnancy and immediately after birth, but also into later childhood. This is crucial to determine safety, but also to disprove the skepticism around vaccines that may derail immunization programs.

"A comprehensive and proactive approach to vaccine safety is needed now more than ever with the hopefully imminent arrival of COVID-19 vaccines," Hviid wrote.

Since the suggested link between the MMR vaccine and risk of autism was put forth in the retracted Wakefield et al. study, several studies have debunked the proposition that vaccinations increase risk of autism; yet, some examining influenza and H1N1 vaccines during pregnancy have been unable to completely reject the hypothesis, Ludvigsson said.

Using Swedish national registry data, Ludvigsson and colleagues conducted a population-based cohort study to examine the risk of autism in children whose mothers received the 2009-2010 swine flu vaccination during pregnancy.

Ludvigsson's group collected data on singleton live births in Sweden when much of the population was immunized in a swine flu pandemic vaccination campaign. They linked vaccination data to national patient registers, including participants from seven healthcare regions in Sweden.

In their primary analysis, researchers compared the risk of autism spectrum disorder in children who were exposed to the swine flu vaccine in utero to those who were unexposed. The secondary outcome was autistic disorder. Researchers also compared the risks associated with vaccination in the first trimester to other periods of pregnancy, and conducted a sensitivity analysis examining maternal epilepsy, and neurologic and psychiatric disease.

The group controlled for maternal age at delivery, BMI, parity, smoking, country of birth, disposable income, health care region, comorbidities, infant sex, and prenatal study time.

In a cohort of more than 69,000 study participants, nearly 40,000 infants were prenatally exposed to the H1N1 vaccine. Mothers of vaccine-exposed infants were older at delivery and had a higher disposable income, and were less likely to have higher BMI, to have smoked, or to have been born outside of Sweden.

In an average seven-year follow-up, researchers found that 1.0% (394) of children who were prenatally exposed to the H1N1 vaccine were diagnosed with autism spectrum disorder, compared to 1.1% (330) of children who were unexposed.

Compared to unexposed children, there was no increased risk of ASD in those exposed to the swine flu shot (adjusted hazard ratio 0.95, 95% CI 0.81-1.12). There was also no increase in risk for the secondary outcome, AD, in children prenatally exposed to the vaccine (aHR 0.96, 95% CI 0.80-1.16). The sensitivity analyses did not change risk estimates.

Ludvigsson and colleagues recognized that their analysis may be limited by a lack of information on H1N1 infection in pregnant women, and may be subject to additional unmeasured confounding.

The researcher also said that understanding vaccine safety in the pregnant population is significant, especially during the COVID-19 pandemic.

"All vaccination research now is important since it increases our understanding about how vaccines will, and will not, influence fetal development," Ludvigsson stated. "That is crucial in anticipation of the coming covid-19 vaccines when likely thousands if not millions of pregnant women will be offered vaccinations against covid-19."

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