Objective: The objective of this study was to determine the population-based incidence of influenza-associated serious neurologic events in children < 5 years of age.

Methods: We conducted a retrospective cohort of children < 5 years of age enrolled in a Medicaid program during the 2016-2017 through 2019-2020 influenza seasons. Serious influenza-associated neurologic events were defined as a neurologic event resulting in hospitalization. Population-based incidence of serious influenza-associated neurologic events was calculated by dividing the number of events by the total accrued follow-up time and expressed per 100,000 influenza person-weeks. Incidence estimates were stratified by neurologic event category, age, sex, neurologic comorbidity, influenza season, and antiviral use.

Results: A total of 79,727 influenza cases among 70,258 unique children were included. The overall incidence of serious influenza-associated neurologic events was 38.0 (95% CI 27.5-51.2) per 100,000 person-weeks of influenza. The most common serious neurologic event was seizure (34.5 per 100,000 influenza person-weeks of influenza, 95% CI 24.5-47.1) whereas encephalitis and ataxia/movement disorders were least common (0.9 per 100,000 influenza person-weeks of influenza, 95% CI 0.02-4.9). Incidence rates were generally higher in children <2 years and significantly more common in children with neurologic conditions compared to those without neurologic conditions. Incidence rates were similar among those with and without influenza antiviral use.

Conclusions: Our findings reveal that serious neurologic complications are uncommon in young children with influenza but markedly higher in those with underlying neurologic conditions. These data emphasize the importance of preventing, identifying, and treating influenza in this vulnerable population.