Humoral immunity in elderly persons vaccinated against influenza shortly before cancellation of COVID-19 pandemic

Influenza-related morbidity and mortality are disproportionately high among older population. Over 2004-2017, the highest proportion of influenza A(H3N2) virus in the WHO European Region was observed in the age group of ≥ 65 years, i.e., 10%. The efficiency of influenza vaccination is shown to be reduced among individuals over 65-70 years of age, due to age-related immune dysfunction (immunosenescence). It may be associated with an imbalance in effector memory T cells and regulatory responses. In this age group, high-dose or adjuvanted influenza vaccines are potentially more effective in terms of hospitalization days and economy than with non-adjuvanted influenza vaccines used at standard doses. The aim of our study was to investigate the ability to form a protective titer of antibodies to influenza virus among elderly persons after vaccination by the end of the COVID-19 pandemic. A total of 31 participants over 60 years of age took part in the study, who were immunized with an influenza quadrivalent inactivated subunit adjuvant vaccine by intramuscular injections. Antibodies to influenza virus strains were determined by performing a hemagglutination inhibition (HI) test one month after the vaccination. For individuals over 60 years old, one month after vaccination, a statistically significant increase in the seroprotection level (p < 0.05) was observed in relation to three strains: A/Victoria/2570/2019(H1N1) pdm09 (up to 74.2%), A/H3N2/Darwin/9/2021 (93.2%), and B/Austria/1359417/2021 (up to 74.2%). The seroprotection level to the B/Phuket/3073/13 strain was 35.5%. The geometric means of antibody titer (GMT) in older individuals before vaccination was 15.1 (log₂ 3.91±0.59) for the H1N1 strain versus 73.7 (log₂ 6.20±0.93) after vaccination; for the H3N2 strain, 52.7 (log₂ 5.72±0.97) and 147.4 (log₂ 7.20±1.22), respectively; for the B/Yamagata strain, 8.6 (log₂ 3.11±0.54) versus 24.1 (log₂ 4.59±0.79). The GMT level for the B/Victoria strain, was 10.1 (log₂ 3.33±0.38) versus 63.0 (log₂ 5.98±0.69) after vaccination. The seroconversion rate (SCR) significantly exceeded the required level of 2.00 for all strains tested (p < 0.05). For both H3N2 and B/Yamagata strains, the GMT was 2.8; for H1N1 and B/Victoria strains, 4.89 and 6.26, respectively. The seroconversion rate for H3N2 and B/Yamagata strains was 41.9%; for the H1N1 strain, 61.3%, for the B/Victoria strain it was 77.4%. The immunogenicity of each component of the influenza vaccine following a single intramuscular immunization of volunteers over 60 years old met at least one criterion of the requirements for inactivated influenza vaccines.

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