Since COVID-19 situation in Thailand, there is no research study of the immune responses to SARS-CoV-2 among the active COVID-19 patients and their high-risk closed-contacts individuals. The aim of this project was to study the antibody production (humoral immune responses) and the immune response mediated by T cells (cell-mediated immune responses) specific to SARS-CoV-2. The results of this study can be used as the knowledge for vaccine development for COVID-19 and the production of a monoclonal antibody for the treatment of infection in stead of using plasma-derived antibodies from the convalescent patients. Moreover, this project has evaluated the antibody production in the individuals who received the inactivated vaccine and viral vector vaccine. During that period, most of the Thai population have been vaccinated with the inactivated vaccine. The knowledge of immune responses from the inactivated vaccine is in need and should be studied as fast as possible. The results from this study showed that the best suitable technique for evaluating the immune responses among many participants is the determination of total antibody specific to spike protein by using ELISA assay. Although the detection of neutralizing antibody is better for evaluation of the protection against SARS-CoV-2 infection, this technique is expensive and low in the feasibility in lab. As for the evaluation of T-cell mediated immune responses by using ELISpot assay, this test cannot be used to evaluated T-cell responses in the individuals who received the inactivated vaccine. We found that the determination of T-cell responses among individuals who got infected with SARS-CoV-2 and individuals who received the viral vector vaccine should be stimulated with peptide from nucleoprotein and spike for ELISpot assay, respectively which does not need to use the whole protein for evaluating T-cell responses by this technique. This project provides the information of the seroprevalence among the high-risk closed-contacts individuals but not getting infected by evaluating the antibody production. We found that the seroprevalence rate among these participants is zero. Although they were identified as the closed contact, but they may not get the viral transmission, so they do not have any antigen to stimulate the immune responses. For the evaluation of the kinetics of antibody production in the individuals who received 2 doses of the inactivated vaccine, the results showed that the level of total antibody is significantly decreased after receiving the 2nd dose for a month. This could be used in the policy for providing the booster dose to them since, the gradually decrease in the antibody would not be enough to fight against SARS-CoV-2 transmission. When compared the antibody level to the individuals who received viral vector vaccine, it revealed that the viral vector vaccine is able to stimulate the higher level of antibody than the inactivated vaccine. The output obtained from this study is the knowledge of the immune response to SARS-CoV-2 in the active COVID-19 patients, the closed-contact individuals, individuals receiving the inactivated vaccine and individuals receiving the viral vector vaccine. As for the outcome and the impact, we found that the determination of the total antibody to spike is suitable for using as a test for screening the immune responses to SARS-CoV-2 resulting to the planning of the policy for the prevention of viral transmission and the recommendation for the vaccination. This result could be used for making the decision of the policy involving the prevention of SARS-CoV-2 spreading, for examples, the booster dose of the COVID-19 vaccine should be considered for the individuals who received the inactivated vaccine. In addition, the booster vaccination should be the viral vector vaccine rather than the inactivated vaccine since this study showed that the inactivated vaccine cannot induce T-cell responses and stimulate the lower level of antibody production compared to the viral vector vaccine. This would be the benefit for the policy of effective prevention of SARS-CoV-2 spreading in Thailand. Additionally, this data can be used as the information for vaccine development to prevent or control COVID-19. This project will lead to outcomes and impacts as follows: In the policy terms, the results of this study can be used to make decisions about the COVID-19 situation in Thailand in term of the stringency or easing of policy for preventing the spread of COVID-19 in Thailand. Based on the results of the analysis of immune responses mediated by the antibody production (Humoral immune responses) and T-cell responses. (Cell-mediated immune responses), these results can be used for developing the test kit for COVID-19 or developing the vaccines in the future. Additionally, the results of this study can be used to understand the immune responses against the SARS-CoV-2 infection that causes COVID-19.