Vaccine Effectiveness Against Influenza-Associated Hospitalizations Among Adults, 2018–2019, US Hospitalized Adult Influenza Vaccine Effectiveness Network

Abstract

We estimated vaccine effectiveness for prevention of influenza-associated hospitalizations among adults during the 2018-2019 influenza season. Adults admitted with acute respiratory illness to 14 hospitals of the US Hospitalized Adult Influenza Vaccine Effectiveness Network and testing positive for influenza were cases; patients testing negative were controls. Vaccine effectiveness was estimated using logistic regression and inverse probability of treatment weighting. We analyzed data from 2863 patients with mean age of 63 years. Adjusted VE against influenza A(H1N1)pdm09-associated hospitalization was 51% (95%CI 25, 68). Adjusted VE against influenza A(H3N2) virus-associated hospitalization was −2% (95%CI −65, 37) and differed significantly by age, with VE of −130% (95% CI −374, −27) among adults 18 to ≤56 years of age. Although vaccination halved the risk of influenza-A(H1N1)pdm09-associated hospitalizations, it conferred no protection against influenza A(H3N2)-associated hospitalizations. We observed negative VE for young-and middle-aged adults but cannot exclude residual confounding as a potential explanation.

Relative Effectiveness of Cell-Cultured versus Egg-Based Seasonal Influenza Vaccines in Preventing Influenza-Related Outcomes in Subjects 18 Years Old or Older: A Systematic Review and Meta-Analysis

“…Finally, other authors have described the modifying effect of previous exposure by the birth cohort that may result in a negative interaction between vaccination with an unmatched strain and imprinted immunity [ 59 ]. This situation has been proposed to explain the age variability of estimated influenza vaccine effectiveness during the mixed 2018–2019 season [ 60 ].…”

Comparison of influenza and COVID-19 hospitalisations in British Columbia, Canada: a population-based study

“…Before Omicron emerged, all homologous or heterologous mRNA and/or ChAdOx1 two-dose schedules were associated with ≥90% reduction in SARS-CoV-2 hospitalisation risk for up to 5–7 months13–15 while vaccine effectiveness against hospital admission due to the Omicron variant at 3 months or longer fell to 54%–55% 16 17. However, vaccine effectiveness against influenza hospitalisation has historically varied by age and ranged from negative values (when vaccine components were antigenically distinct from the epidemic strain) to ~80% (in case of high match) 18–21. During the course of the COVID-19 pandemic, vaccination status of the Canadian population dramatically shifted from 100% unvaccinated in 2020 to over 90% of those vaccinated with at least one dose (aged 12 years and over) by the end of 2021.…”

Association of Online Search Trends With Vaccination in the United States: June 2020 Through May 2021

“…In particular, influenza vaccinations are a common and long-term applied prevention measure with a well-known side effect profile ( 36 ). Although direct comparisons of influenza and COVID-19 vaccinations are not available, the efficacy of mRNA COVID-19 vaccines appears largely higher when compared to influenza vaccines ( 37 – 39 ). However, influenza vaccines are shown to reduce influenza-related hospitalizations and mortality despite a lower vaccine efficacy in older individuals (≥65 years of age) ( 40 ).…”