Background: Monovalent 2009 H1N1 influenza vaccines were licensed and administered in the United States during the H1N1 influenza pandemic between 2009 and 2013.

Methods: Vaccine Adverse Event Reporting System received reports of adverse events following immunization (AEFI) after H1N1 vaccination. Selected reports were referred to the Centers for Disease Control and Prevention's Clinical Immunization Safety Assessment network for additional review. We assessed causality using modified World Health Organization criteria.

Results: There were 3,928 reports of AEFI in children younger than age 18 years after 2009 H1N1 vaccination received by January 31, 2010. Of these, 214 (5.4%) were classified as serious nonfatal and 109 were referred to Clinical Immunization Safety Assessment for further evaluation. Ninety-nine (91%) had sufficient initial information to begin investigation and are described here. The mean age was 8 years (range, 6 months-17 years) and 38% were female. Median number of days between vaccination and symptom onset was 2 (range, -11 days to +41 days). Receipt of inactivated, live attenuated, or unknown type of 2009 H1N1 vaccines was reported by 68, 26 and 5 cases, respectively. Serious AEFI were categorized as neurologic events in 47 cases, as hypersensitivity in 15 cases and as respiratory events in 10 cases. At the time of evaluation, recovery was described as complete (61), partial (16), no improvement (1), or unknown (21). Causality assessment yielded the following likelihood of association with 2009 H1N1 vaccination: 8 definitely; 8 probably; 21 possibly; 43 unlikely; 17 unrelated; and 2 unclassifiable.

Conclusions: Most AEFI in children evaluated were not causally related to vaccine and resolved without sequelae. Detailed clinical assessment of individual serious AEFI can provide reassurance of vaccine safety.