Flu vaccines prevented the majority of critical and life-threatening influenza cases among children during the 2019-2020 season, a case-control study found.

Vaccine effectiveness was 63% (95% CI 38-78%) against “critical influenza” and 75% (95% CI 49-88%) against “life-threatening influenza,” reported Manish Patel, MD, of the CDC, and colleagues in Clinical Infectious Diseases.

CDC Director Rochelle Walensky, MD, praised the results of this study in a statement.

“Flu season has started and currently flu vaccination is down in children,” she said. “This study highlights that flu can cause serious illness in children, but flu vaccines can be lifesaving.”

CDC also noted a record-breaking 199 pediatric deaths from flu during the 2019-2020 flu season, when most flu viruses were caused by influenza A (H1N1) pdm09 and B-Victoria strains that were “antigenically different” from the strains in the flu vaccine.

Patel and colleagues examined data from 17 hospitals from December 2019 to April 2020 for children who were admitted to the ICU with acute respiratory infection. “Critical” admissions were defined as those to a pediatric ICU with at least one sign of systemic illness and acute respiratory infection, including the need for mechanical ventilation.

The test-negative design enrolled case patients who tested positive for influenza via RT-PCR and controls with critical acute respiratory infection who tested negative. Cases and controls were matched by hospital and age group (ages 2 and younger, ages 2-8, and ages 9-17).

Vaccination status was determined by state immunization registries or patient medical records, including those from the child’s pediatrician, as well as verbal reports of vaccination with dates and location that were “considered plausible.”

Of the initial 329 patients, 160 were fully vaccinated, 38 were partially vaccinated, and 131 were unvaccinated. Overall, 291 patients were included in the analysis, including 159 cases and 132 controls. Median age was 6 for case patients, and 4 for controls. Most patients had underlying health conditions, with respiratory or neurologic conditions being the most common.

For case patients, median length of ICU stay was 2 days, and median hospital stay was 5 days. Fifty-seven patients (36%) had life-threatening illness, and of these, 88% required invasive mechanical ventilation, 58% needed vasopressor-dependent shock, and four died.

Among sequenced cases, half were influenza A (H1N1) pdm09, and 38% were B-type viruses, most B-Victoria. Vaccine effectiveness was 64% against critical illness from influenza A (H1N1) and 68% against B-lineage viruses.

Against mismatched A/H1N1 viruses, vaccine effectiveness was 47% (95% CI -21 to 77%), though Patel and colleagues noted that the confidence intervals were overlapping. Effectiveness against mismatched B-Victoria viruses was higher, at 75% (95% CI 37-90%), they added.

Limitations to the study included the fact that child hospitalizations for influenza are relatively rare, as well as its observational nature. Patel and colleagues also cited potential misclassification of vaccination status or case/control status.

CDC noted that the Advisory Committee on Immunization Practices (ACIP) recommends a flu shot every year for all individuals ages 6 months and up, but recent estimates indicate that only 51% to 68% of children were vaccinated against the flu during the 2020-2021 flu season.

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    Molly Walker is deputy managing editor and covers infectious diseases for MedPage Today. She is a 2020 J2 Achievement Award winner for her COVID-19 coverage. Follow

Disclosures

This work was supported by the CDC under a contract to Boston Children’s Hospital.

The findings and conclusions of this report are those of the authors and do not necessarily represent the official position of the CDC.

Olson and Patel are employed by the CDC.

Other co-authors disclosed support from the CDC, NIH, National Heart, Lung, and Blood Institute, American Thoracic Society, American Lung Association, Kiadis, La Jolla Pharmaceuticals, the Eunice Kennedy Shriver National Institute of Child Health and Human Development, UpToDate, the International Sepsis Forum, Families Fighting Flu, Merck, Pennsylvania Department of Health, Sanofi, Quidel, and Genentech.

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