Aim: Regarding coronavirus disease (COVID-19), despite antibody measurement alone providing insufficient data, studies evaluating immune responses to vaccination or disease have focused on humoral immunity. Uncoupled humoral and cellular responses may be present after vaccination or disease. In this study, the COVID-19 recovered individuals (CRI) and vaccinated healthcare workers (VHCW) were classified into two groups according to immunoglobulin G (IgG) levels to SARS-CoV-2 spike antigen, then cellular immunity was evaluated with interferon-gamma (IFN-γ).

Material and Methods: The CRI group (n=30) had COVID-19 and were not vaccinated. The VHCW group (n=47) had two doses of CoronaVac and was never infected. In VHCW, humoral response and IFN-γ were evaluated one month after vaccination, while blood samples were taken in recovered patients between one month and one year after infection.

Results: In the VHCW group, IFN-γ (p=0.848), and age (p=0.949) were similar in IgG<7 and IgG≥7 subgroups. No correlation was present between IFN-γ and IgG levels in VHCW (p=0.711). In the CRI group, IFN-γ and age were higher in the subgroup of IgG≥7 (p=0.005, p<0.001, respectively). There was no statistically significant correlation between IFN-γ and IgG in the CRI group; however, there was a trend (p=0.057, r=0.35). No difference was observed in terms of IgG levels between the VHCW and CRI groups; while IFN-γ was higher in the CRI group (p<0.001).

Conclusion: Demonstrating the immune response to COVID-19 is important for the development of vaccines and therapeutics. Evaluating cellular response (T cell response) to vaccines is worthy when making public health decisions during pandemics.