Influenza Vaccine for 2023-24 Season Is Effective Against Hospitalization

The 2023-24 seasonal influenza vaccine decreased the risk for medically attended influenza-associated outpatient visits and hospitalization in children, adolescents, and adults, according to study findings reported in the Morbidity and Mortality Weekly Report (MMWR).

MMWR published 2 sets of interim estimates of 2023-2024 seasonal influenza vaccine effectiveness (VE) against laboratory-confirmed influenza — one for the US as a whole and the other for California.^1,2 The national report, which reported on VE in the outpatient and inpatient setting, was based on 22 US states in data obtained from 4 VE networks affiliated with the Centers for Disease Control and Prevention.¹

According to the national interim estimate, VE for any influenza-associated acute respiratory illness (ARI) in children and adolescents aged 6 months to 17 years ranged from 59% to 67% in the outpatient setting and VE against influenza-related hospitalization ranged from 52% to 61%. For influenza A, VE ranged from 46% to 59% in the outpatient setting and 46% to 56% against influenza-associated hospitalization. For influenza B, VE ranged from 64% to 89% in the outpatient setting.¹

VE against influenza-associated ARI in adults at least 18 years of age ranged from 33% to 49% in the outpatient setting and VE against influenza-related hospitalization ranged from 41% to 44%. For influenza A, VE was 27% to 46% in the outpatient setting and 40% to 42% against influenza-associated hospitalization. For influenza B, VE was 78% in 2 networks in the outpatient setting and 60% against influenza-associated hospitalization.¹

In adults aged 18 to 64 years, VE against influenza-associated ARI ranged from 25% to 52% in the outpatient setting and 40% to 49% against influenza-associated hospitalization. Among adults at least 65 years of age, VE against influenza-associated ARI ranged from 41% to 51% in the outpatient setting and was 42% against any influenza-associated hospitalization in 2 networks.¹

Among several limitations of this US interim report, small sample sizes prevented estimation of VE for some age groups and settings, and patients who received at least 1 dose of 2023-24 influenza vaccine were considered vaccinated, although children aged 6 months to 8 years are recommended to receive 2 influenza vaccine doses if they were not previously vaccinated.¹

“These findings indicate that the 2023-24 seasonal influenza vaccine is effective at reducing the risk of influenza-associated outpatient visits and hospitalization,” stated authors of the MMWR interim report on US influenza VE. “All eligible persons aged ≥6 months should receive annual influenza vaccination.”¹

In a separate study, also published in MMWR, investigators reported interim findings of influenza VE from California from October 1, 2023, to January 31, 2024.²

A total of 678,422 influenza laboratory test results that met the inclusion criteria were reported to the California Department of Public Health (CDPH) in this period, including 77,501 (11%) positive and 600,921 (89%) negative test results.²

The overall adjusted VE was 45% against having a positive influenza A or B test result, 42% against a positive influenza A test result, and 76% against a positive influenza B test result. VE was highest in individuals less than 18 years of age (56%), 48% in those aged 18 to 49 years, 36% in those aged 50 to 64 years, and 30% in those aged 65 years and older. An age gradient was observed for influenza A and B; however, the VE for individuals aged 65 years and older for influenza B (54%) was greater compared with influenza A (29%).²

The researchers noted that 2023 was the first year in which reporting negative influenza test findings to the CDPH and influenza vaccination to a centralized immunization information system was mandated.²

“Interim VE estimates from this analysis indicate that the current seasonal influenza vaccine protects against receipt of a positive influenza laboratory test result among persons aged ≥6 months,” stated authors of the MMWR interim report on California influenza VE.²

Disclosure: Some of the authors in the study by Frutos et al declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of authors’ disclosures.

This article originally appeared on Pulmonology Advisor