Immunogenicity and safety of quadrivalent influenza vaccine among adults in Tianjin, China, with a focus on age as a risk factor

Abstract Background Older adults are more vulnerable to seasonal influenza than younger adults. The immune responses of elderly people to the influenza vaccine are usually poorer than those of young people. We conducted a study to evaluate the immunogenicity and safety of a quadrivalent inactivated influenza vaccine (IIV4) and the influence of age among adults in Tianjin. A total of 167 healthy adults in Tianjin were recruited and administered a single dose of IIV4 from October 2021 to March 2022. Blood samples were collected to assay hemagglutination inhibition (HAI) antibody titers before and 21 to 28 days after vaccination. Safety information was collected for 28 days and 6 months after vaccination. Differences in immunogenicity and safety were compared among different age cohorts, and multivariate logistic regression was used to estimate the effect of age and other factors on seroconversion and seroprotection. Results Overall, the geometric mean titers (GMTs) for all strains in the ≥ 65 age group were lower than those in the < 65 age group, whereas the seroconversion rates (SCRs) were similar. Multivariate logistic regression analysis showed that age, influenza vaccination history, and baseline HAI antibody titer were the main independent factors affecting seroconversion and seroprotection. Higher age was an unfavorable factor for seroconversion for H1N1 (OR, 0.971; 95% CI: 0.944–0.999; P = 0.042) and B/Victoria (OR, 0.964; 95% CI: 0.937–0.992; P = 0.011). All adverse events (AEs) were grade 1 or grade 2, occurred within 28 days after vaccination, and no serious adverse events occurred. Conclusions IIV4 was well-tolerated and immunogenic in healthy adults in Tianjin. Our findings also confirmed that age is an independent factor associated with a lower immune response to IIV4.