Duration of Illness in Children Younger Than 18 Years Dying of Influenza in California: 2004–2023

Annually, thousands of children in the United States are hospitalized and 35–200 die from influenza illness and its complications.1,2 Influenza vaccines are highly effective against life-threatening influenza in children.3 Even a partially effective influenza vaccine can avert symptomatic illness, hospitalization, and death and reduce illness severity if a child is infected. The Advisory Committee on Immunization Practices began recommending annual influenza vaccination of all children aged at least 6 months starting in 2010–2011. Vaccination rates among children younger than age 18 years vary, ranging from 33.4% to 62.6% during 2010–2019.4,5 Children who die from influenza often have an underlying condition known to increase the risk of influenza-associated complications and are unvaccinated.6 For children with severe influenza or illness that can result in hospitalization or death, previous vaccination at least 2 weeks before illness reduces disease severity and risk of death.3 We described patient, vaccination, and influenza illness characteristics by underlying condition status among children younger than age 18 years who died from influenza from 2004 to 2023 in California.California law requires reporting pediatric influenza-associated deaths to public health authorities. This activity was reviewed by the Centers for Disease Control and Prevention (CDC), deemed not research, and was conducted consistent with applicable federal law and CDC policy (see, for example, 45 CFR part 46, 21 CFR part 56, 42 USC §241(d), 5 USC §552a, and 44 USC §3501 et seq). We included all pediatric influenza-associated deaths reported from October 9, 2004, to September 30, 2023. The race and ethnicity of patients with fatal pediatric influenza were determined by electronic health records and included for descriptive purposes. The duration of illness before death was calculated as the difference between symptom onset date and date of death. Clinical variables of interest included whether a patient was admitted to the intensive care unit, use of antibiotics, use of antivirals, type of medical facility, secondary bacterial infection status, pneumonia status, vaccination status, number of underlying conditions, location of death, and influenza type. The number of underlying conditions was determined by the total number of conditions known to have an association with severe influenza outcomes, including respiratory, metabolic, cardiac, renal, pulmonary, neurologic, and immunocompromising conditions. Clinical variables were determined from chart review and entered into the electronic health record.During 2004–2023, 0–46 children died from influenza and were reported to public health authorities. There was a significant drop in deaths from 2020 to 2021 because of the COVID-19 pandemic. Among the 245 children who died of influenza, the median age was 6.3 years (range, 3 days–17 years) (Table 1). Most (59%; n = 145) were infected with influenza A and 18% (n = 45) with influenza B. Fewer than 1% (n = 1) were coinfected with A and B; 22% (n = 54) had an unknown influenza type. Most children (62%; n = 152) had at least 1 underlying condition (median, 1; range, 1–6). Of children with known vaccination status, 27% (37/135) had documented vaccination at least 2 weeks before illness onset, and 29% (27/92) of children with underlying conditions were vaccinated vs 23% (10/43) of previously healthy children. Many children died in the intensive care unit (58%, n = 142/245). Most children with underlying conditions had pneumonia (55%) and frequently received antiviral treatment (53%) (Table 1). The median duration of illness before death was 7.5 days for children with preexisting conditions and 4.0 days for healthy children. Previously healthy children accounted for 38% of the influenza deaths in California.Our findings demonstrate that some children with influenza can have rapid evolution to severe disease; although counterintuitive, this phenomenon is most striking in previously healthy children. More than 50% of children without underlying conditions died at home or in an emergency department; only 25% received antiviral treatment. The vaccination coverage of children dying of influenza in California was low compared with national estimates for children aged 6 months–17 years, which range between 43.7% and 63.7% depending on the influenza season.6,7 This analysis is limited by unknown vaccination status for some children, because reporting vaccine doses to an immunization registry was not mandatory in California until 2023; data before 2023 were gathered from multiple sources to improve data quality.This analysis highlights potential missed opportunities for the prevention of fatal influenza illness among children with and without underlying conditions. Future investigation should assess effects of vaccination on influenza illness severity while accounting for other factors, such as prior season vaccination, severity of preexisting conditions, and propensity to seek health care. Vaccination is recommended for all children aged at least 6 months, and messaging directed at both clinicians and parents to support increased influenza vaccination is vital to reducing influenza burden.We thank the local health departments in California for conducting all case investigations for the deaths described in this analysis.