CDC Reports Neurologic Complications of Flu in Kids This Year

During the current flu season, influenza-associated encephalopathy or encephalitis (IAE) was identified in 9 of 68 (13%) pediatric deaths, including four with acute necrotizing encephalopathy (ANE), a severe form IAE, CDC researchers reported.

In comparison, the median proportion of pediatric influenza deaths with IAE during the 2010-11 through 2024-25 flu seasons was 9% (166 of 1,840), reported Amara Fazal, MD, of the CDC, and colleagues in Morbidity and Mortality Weekly Report. It is not known whether cases observed this season vary from expected numbers, they noted.

In response to anecdotal reports the agency received in late January of children with influenza-associated ANE, including several fatal cases, CDC examined influenza-associated pediatric deaths for trends of the proportions of cases with IAE, Fazal and colleagues wrote.

Earlier this month, MedPage Today reported that public health officials were looking into reports of a small potential uptick in neurologic complications of influenza in children — particularly ANE. At the time, a spokesperson for the CDC told MedPage Today that the agency “is aware of anecdotal reports” of influenza A-associated ANE in children. “We do not have active surveillance for ANE, but we are looking into the situation and will provide updates.”

Fazal and colleagues wrote that on Feb. 24, “CDC posted a national call for possible cases of pediatric IAE identified during this influenza season (Oct. 1, 2024, through May 30, 2025) on the Epidemic Information Exchange (EPI-X).”

Among the three major IAE syndromes that have been described, ANE is the most severe form and is associated with high rates of long-term neurologic sequelae and death.

“Criteria for diagnosis of ANE, including influenza-associated ANE, are well characterized and include febrile illness preceding or concurrent with the onset of neurologic signs or symptoms, rapid neurologic decline, and neuroimaging demonstrating symmetric lesions affecting the bilateral thalami and other parts of the brain,” the researchers explained.

During the current flu season, all four ANE deaths occurred in children less than 5 years of age and they all had laboratory-confirmed influenza A(H1N1)pdm09; one child had underlying medical conditions.

Two of the children had been vaccinated against influenza more than 2 weeks before illness onset, and two had not been vaccinated during the current season. Two of the children received oseltamivir (Tamiflu), two experienced seizures during hospitalization, and all received mechanical ventilation.

Among pediatric influenza-associated deaths during the 2010-11 through 2024-25 flu seasons, yearly percentages with IAE ranged from 0% (2020-21 season) to 14% (2011-12 season). IAE prevalence was highest among children 2-4 years old (10%) and lowest among those less than 6 months of age (5%). Fifty-two percent of the children were female, and 40% were non-Hispanic white. Seventy-two percent of the children had influenza A, and 28% had influenza B. Among 73 influenza A cases with available subtype, 56% were A(H1N1)pdm09 and 44% were A(H3N2).

There were no underlying medical conditions in 54% of the children. Twenty percent of the children had received one or more doses of the current season influenza vaccine more than 2 weeks before illness onset, and 73% received antiviral treatment.

Ultimately, 93% of the children required mechanical ventilation. Other acute complications included acute respiratory distress syndrome (34%), pneumonia (33%), and sepsis (28%). Most (96%) children died during hospitalization, while 2% died in the emergency department, and 2% died outside the hospital setting.

“Enhanced surveillance to systematically identify and report pediatric IAE cases, including ANE, in the United States during the remainder of the 2024-25 season would improve understanding of the incidence of this influenza complication and the frequency of severe outcomes, including long-term neurologic sequelae or death,” wrote Fazal and colleagues.

Study limitations included that data might have over- or underestimated IAE prevalence, and that data on IAE prevalence among pediatric influenza-associated deaths during the current flu season are preliminary and based on small numbers. Also, IAE prevalence among pediatric influenza-associated deaths is likely not representative of overall pediatric IAE prevalence, especially less severe cases. And due to the lack of established surveillance for IAE in the U.S., it was not possible to determine whether anecdotal reports of hospitalizations and deaths due to IAE — including ANE — during the current flu season are within or above expected ranges.

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