Summary: Study refutes any association between the H1N1 vaccine given to pregnant women and an increased risk of autism diagnosis in children.
Source: Karolinska Institute
Two recent studies were unable to rule out that H1N1 (“swine flu”) vaccination (“Pandemrix”) and seasonal influenza vaccination given to pregnant women might be associated with autism-spectrum disorder in the offspring. Now, a large study by researchers at Karolinska Institutet in Sweden, published in the journal Annals of Internal Medicine, refutes any such association.
Autism spectrum disorder is a severe neurodevelopmental childhood disorder characterized by impaired communication, lack of social skills and repetitive behavior. The disease has its onset in childhood.
While some studies indicate that influenza vaccination during pregnancy protects against morbidity in both the woman and her offspring, the long-term risks of H1N1 vaccination during fetal life have not been examined in detail. However two recent studies were unable to rule out that offspring to women undergoing influenza or H1N1 influenza vaccination during pregnancy, and especially during the first trimester, were at increased risk of autism-spectrum disorder.
Researchers from Karolinska Institutet, linked vaccination data in pregnant women from seven Swedish healthcare regions in 2009-2010 to the Swedish Medical Birth Register and the Swedish National Patient Register to identify autism-spectrum disorder in the offspring.
The importance of vaccination research
Of the 39,726 vaccine-exposed children, 394 (cumulative incidence, 1.0%) had a diagnosis of autism-spectrum disorder during the six-year follow-up compared with 330 (1.1%) among 29,293 unexposed children. Adjusting for potential confounders, H1N1 vaccine exposure during fetal life was not associated with a later childhood diagnosis of autism-spectrum disorder (adjusted hazard ratio=0.95; 95%CI=0.81-1.12). Results were similar for vaccinations in the first pregnancy trimester.
“Our null findings are important since some people have suspected that vaccinations could cause autism, and the anti-vaccine movement seems to be growing in the Western world,” says lead author, Professor Jonas F Ludvigsson, pediatrician at Örebro University Hospital and professor at the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet. “H1N1 vaccination has previously been linked to an increased risk of narcolepsy in young people, but vaccinating pregnant women does not seem to influence the risk of autism-spectrum disorder in the offspring.”
He continues: “Vaccination research has never been more important. Anticipating a vaccine against COVID-19, millions of pregnant women are likely to be offered such a vaccination. While our research group did not study COVID-19 vaccine effects, our research on H1N1 vaccination adds to the current knowledge about vaccines, pregnancy and offspring disease in general.”
Adjusted for other factors
The researchers adjusted their analyses for such confounders as maternal smoking, height-weight, maternal age and comorbidity in order to minimize the influence of other factors that might explain any association between vaccination and autism.
“Without taking such factors into consideration, so-called confounding may create spurious associations that do not reflect a true association,” adds co-author, Ass. Prof. Bjorn Pasternak, Department of Medicine, Karolinska Institutet (Solna).
Funding: This project was supported by grants from the Swedish Research Council, and the Swedish Council for Working Life and Social Research. Dr Pasternak was supported by the Strategic Research Area Epidemiology program at Karolinska Institutet and the Swedish Research Council.
Dr Ludvigsson coordinates a study on behalf of the Swedish IBD quality register (SWIBREG). This study has received funding from the Janssen corporation.
About this autism research article
Source: Karolinska Institute Contacts: Press Office – Karolinska Institute Image Source: The image is in the public domain.
Background: There are concerns that influenza vaccine exposure during pregnancy may be associated with increased risk for autism spectrum disorder (ASD).
Objective: To examine the risk for ASD in offspring of mothers who were vaccinated against influenza A(H1N1)pdm09 (“swine flu”) during pregnancy.
Design: Population-based cohort study using nationwide registers.
Setting: Seven health care regions in Sweden.
Participants: Live births between October 2009 and September 2010, with follow-up through December 2016. In total, 39 726 infants were prenatally exposed to H1N1 vaccine (13 845 during the first trimester) and 29 293 infants were unexposed.
Measurements: Cox regression was used to estimate hazard ratios (HRs) for the primary outcome, ASD, before and after adjustment for potential confounders. The secondary outcome was autistic disorder (AD).
Results: Mean follow-up was 6.7 years in both unexposed and exposed children. During follow-up, 394 (1.0%) vaccine-exposed and 330 (1.1%) unexposed children had a diagnosis of ASD. In adjusted analyses, prenatal exposure to H1N1 vaccination was not associated with a later diagnosis of ASD (adjusted HR [aHR], 0.95 [95% CI, 0.81 to 1.12]) or AD (aHR, 0.96 [CI, 0.80 to 1.16]). The 6-year standardized cumulative incidence difference between the unexposed and exposed children was 0.04% (CI, −0.09% to 0.17%) for ASD and 0.02% (CI, −0.09% to 0.14%) for AD. Restricting the analysis to vaccination in the first trimester of pregnancy did not influence risk estimates (aHR, 0.92 [CI, 0.74 to 1.16] for ASD and 0.91 [CI, 0.70 to 1.18] for AD).
Limitation: Data on H1N1 influenza infection are lacking.
Conclusion: This large cohort study found no association between maternal H1N1 vaccination during pregnancy and risk for ASD in the offspring.