Impact of influenza immunity on the mortality among older adults hospitalized with COVID-19: a retrospective cohort study

It has been suggested that the outcomes of coronavirus disease 2019 (COVID-19) are better in individuals having recently received an influenza vaccine than in non-vaccinated individuals. We hypothesized that this association depends on the humoral responses against influenza viruses. We aim to assess the relationship between the humoral immunity against influenza and the 3-month all-cause mortality among hospitalized older patients with COVID-19. We performed an exploratory retrospective study of older patients (aged 65 and over) hospitalized for confirmed COVID-19 between November 2020 and June 2021. Previous humoral responses to influenza viruses were assessed using a hemagglutination inhibition assay on routinely collected blood samples. The study’s primary outcome was the 3-month all-cause mortality, and the secondary outcomes were severe COVID-19 (oxygen requirement ≥ 6 L/min or ventilatory support) and complications (kidney or heart failure, thrombosis and bacterial infection). In the cohort of 95 patients with COVID-19, immunity against influenza vaccine subtypes/lineages was not significantly associated with 3-month all-cause mortality, with an OR [95%CI] of 0.22 [0.02–1.95] (p = 0.174) for the H1N1pdm09 subtype, 0.21 [0.03–1.24] (p = 0.081) for A/Hong Kong/2671/2019 H3N2 subtype, 1.98 [0.51–8.24] (p = 0.329) for the B/Victoria lineage, and 1.82 [0.40–8.45] (p = 0.437) for the B/Yamagata lineage. Immunity against influenza vaccine subtypes/lineages was also not significantly associated with severity and complication. Immunity against influenza subtypes/lineages included in the 2020–2021 vaccine was not associated with a lower 3-month all-cause mortality among COVID-19 hospitalized patients.