The coronavirus disease 2019 (coVid-19) pandemic still continues around the world by making peaks with different variants. in the fight against coVid-19, vaccination is currently the only protective measure. in Turkey, vaccination started firstly in healthcare workers on 13 January 2021 with the coro- naVac (Sinovac Biotech, chinese) vaccine, and booster doses were administered with the coronaVac and comirnaty vaccines in July 2021. in this cross-sectional study, it was aimed to determine the humoral and cellular immunity levels of the employees of the ToBB ETu Hospital 2.5-3.5 months after three doses of vaccination and identify the effective factors. With the power analysis that was conducted with the G*Power software, it was determined to be suitable to include 40 ToBB ETu Hospital workers who had their third dose with the CoronaVac vaccine and 60 workers of the same hospital who had their third dose with the comirnaty (Pfizer Biontech, Germany) vaccine, and age- and sex-matching was conside- red by selecting 60 workers randomly from among 223 workers who had comirnaty as their third dose. The study excluded individuals who had a previous coVid-19 infection or had a positive Pcr test result. After collecting blood samples on 18-22 october 2021, severe acute respiratory syndrome coronavirus-2 (SArS-coV-2) immunoglobulin G levels were studied with the chemiluminescence immunoassay method using a Beckman-Access device (Beckman coulter inc., cA, uSA) for humoral immunity, and T-SPoT test (T-SPoT. coVid SArS-coV-2 test-oxford immunotec, Singapur) was carried out for cellular immunity.Additionally, demographic data, habits, personal information such as weight and height, reasons for choosing the vaccine and post-vaccine side effects were obtained through the data collection form. data collection was completed on 29 november 2021. As a result, among the healthcare workers who had re- ceived their third dose of vaccine with comirnaty and coronaVac; while 25% of those who received three doses of coronaVac had humoral immunity, 93.3% humoral immunity was determined in the heteroge- neous vaccine group with two doses of coronaVac and one dose of comirnaty vaccine (p< 0.05). cellular immunity was found to be higher (88.3%, 70%) in those who received the heterologous vaccine than those who completed three doses of the same vaccine (p< 0.05). in the heterogeneous vaccination gro- up consisting of the healthcare workers who had their third vaccine dose with Comirnaty and those who had it with coronaVac, humoral and cellular immunity levels were found high. Among the participants whose humoral immunity was found negative, cellular immunity was present in 75% of those who had comirnaty as their third dose and 63.3% of those who had coronaVac. While no significant relationship was found between immunity levels and age, sex or body mass index (BMi), the levels of both forms of immunity were lower in those who had chronic diseases. Among the participants, physicians preferred the third dose of comirnaty vaccine at a rate of 71.4% and laboratory workers preferred this vaccine at a rate of 80% according to the workplace. in the third dose vaccine preference reasons, it was stated that 55% would continue with the same vaccine, experience fewer side effects, 44% provide more immu- nity and 9% do not prevent going abroad. The incidence of side effects after the third dose of vaccine was 53% higher in those who received the comirnaty vaccine than in those who received coronaVac (35%). Based on these data, it was concluded that heterologous vaccination should be preferred in vac- cination strategies, and knowing cellular immunity levels is important for the decision. There is a need for more comprehensive and follow-up studies on how long humoral and especially cellular immunity lasts