Effectiveness of Cell Culture-Based Influenza Vaccine, 2023-2024.

Background: The cell culture-based inactivated influenza vaccine (ccIIV) was first approved for use in children aged 4-17 years in 2016 in the United States. The approved age indication for ccIIV was expanded to include children 6 months and older beginning the 2022-2023 season. There is limited real-world data on ccIIV effectiveness in children. We assessed ccIIV effectiveness using the test-negative design during the 2023-2024 season.

Methods: Patients aged 6 months to 64 years who sought outpatient or telehealth care for acute respiratory illness were actively recruited between October 2023 and May 2024. Symptom eligibility criteria included cough, with an illness duration of ≤ 7 days. Respiratory samples were tested by reverse transcription polymerase chain reaction to identify influenza cases. Vaccination dates and products were determined by immunization records. Analyses were restricted to ccIIV recipients and unvaccinated participants. Cell culture-based inactivated influenza vaccine effectiveness was estimated as [100% × (1 - odds ratio)] for vaccination in cases versus test-negative controls, with adjustment for age and calendar time.

Results: Among 1850 participants, 12% were age 6 months to 3 years, 32% were age 4-17 years, and 56% were age 18-64 years. Influenza was detected in 505 (27%) participants; 267 had influenza A/H1N1pdm09; 149 had influenza B; 56 had influenza A/H3N2; and 33 had influenza A with unknown subtype. A total of 470 (25%) received ccIIV. Among children (age 6 months to 17 years), ccIIV effectiveness was 64% [95% confidence interval (CI), 37%-81%] against influenza A/H1N1pdm09, 75% (95% CI, 43%-91%) against influenza B, and 76% (95% CI, 4%-97%) against influenza A/H3N2. Among adults (age 18-64 years), ccIIV effectiveness was 56% (95% CI, 31%-73%) against influenza A/H1N1pdm09, 62% (95% CI, 13%-86%) against influenza B, and 33% (95% CI, -62% to 76%) against influenza A/H3N2. Young children aged 6 months to 3 years had the highest point estimates (88%-98%).

Conclusion: Cell culture-based inactivated influenza vaccine generated substantial real-world effectiveness against medically attended, laboratory-confirmed influenza in children and adults during a season when influenza A/H1N1pdm09 viruses predominated and influenza B and influenza A/H3N2 co-circulated at lower levels.