• Anders Hviid, Henrik Svanström, Ditte Mølgaard-Nielsen, and Philipp Lambach.
• Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark.
• JAMA Pediatr. 2017 Mar 1; 171 (3): 239-248.

ImportanceSeveral studies investigating potential adverse effects of the pandemic A(H1N1) vaccine have supported that influenza A(H1N1) vaccination does not increase the risk for major pregnancy and birth adverse outcomes, but little is known about possible adverse effects in offspring of A(H1N1)-vaccinated mothers beyond the perinatal period and into early childhood.ObjectiveTo evaluate whether pandemic influenza A(H1N1) vaccination in pregnancy increases the risk for early childhood morbidity in offspring.Design, Setting, And ParticipantsRegister-based cohort study comprising all live-born singleton children in Denmark from pregnancies overlapping the A(H1N1) influenza vaccination campaign in Denmark, from November 2, 2009, to March 31, 2010. From a cohort of 61 359 pregnancies, offspring exposed and unexposed to the influenza A(H1N1) vaccine during pregnancy were matched 1:4 on propensity scores.ExposureVaccination in pregnancy with a monovalent inactivated AS03-adjuvanted split virion influenza A(H1N1)pdm09 vaccine (Pandemrix; GlaxoSmithKline Biologicals).Main Outcomes And MeasuresRate ratios of hospitalization in early childhood until 5 years of age. Hospitalization was defined as (1) first inpatient hospital admission, (2) all inpatient hospital admissions, and (3) first hospital contact for selected diseases, which included individual infectious diseases and individual neurologic, autoimmune, and behavioral conditions.ResultsThe mean (SD) age at end of follow-up was 4.6 (0.40) years for the 61 359 children included in the study. In the cohort, the mothers of 55 048 children were unvaccinated, 349 mothers were vaccinated in the first trimester, and 5962 mothers were vaccinated in the second or third trimesters. Children exposed in the first trimester were not more likely to be hospitalized in early childhood than unexposed children (hospitalization rates per 1000 person-years, 300.6 for exposed vs 257.5 for unexposed; rate ratio, 1.17; 95% CI, 0.94-1.45). Similarly, children exposed in the second or third trimester were not more likely to be hospitalized in early childhood than unexposed children (hospitalization rates per 1000 person-years, 203.6 for exposed vs 219.3 for unexposed; rate ratio, 0.93; 95% CI, 0.87-0.99). This 7% decreased risk was primarily a result of reduced risks for infectious disease-related hospitalizations.Conclusions And RelevanceTo our knowledge, this is the most comprehensive study to date of potential adverse effects manifesting after the perinatal period. We detected no increased risk for early childhood morbidity. These results support the safety profile of the influenza A(H1N1) vaccine used in pregnancy.

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