Antibody responses to COVID-19 vaccines in older adults.

Because of the senescence of the immune system, antibody response to the COVID-19 vaccines may differ from older to younger adults. The study aim compares the titers of SARS-CoV-2 IgG antibody of patients ≥60 years who received three doses of CoronaVac vaccine and those who received two doses of CoronaVac+1 dose of Pfizer-BioNTech after 1 month of the last vaccination. Patients ≥60 years who received the CoronaVac vaccine between March 1, 2021, and April 30, 2021, who did not have COVID-19 disease before the first dose of vaccination and were negative for COVID-19 antibodies, whose antibodies were tested before the third dose of vaccination, and who did not have any COVID-19 disease during the follow-up were included. The demographic characteristics and comorbidities of patients were recorded. An immunofluorescence assay (IFA) fast test and a chemiluminescent microparticle immunoassay (Abbott) were used to measure SARS-CoV-2 quantitative antibody levels at the first month after the third-dose vaccine. Totally 81 patients, 41 patients in third dose of the CoronaVac group (female:male 18:23, mean age 69.4 ± 8.5), and 40 patients in third dose of the Pfizer-BioNTech group (female:male 15:25, mean age 69.9 ± 9.1) were included. The patients' comorbidities in the groups were similar. The titers of IgG antibodies to SARS-CoV-2 measured according to both IFA and Abbott Kit at first month the third dose vaccination was significantly higher in the Pfizer-BioNTech group (p ≥ 0.001, p = 0.012, respectively). The results report that the formed immunity in the first month after the two doses of CoronaVac+1 dose Pfizer-BioNTech vaccine was higher than three doses of CoronaVac vaccine in older adults.