Introduction
The worldwide use of effective and safe COVID-19 vaccines is still of high priority to control the pandemic and to reduce the burden of COVID-19. The vaccine type and vaccination schedule affect many of the cellular and molecular elements of innate and adaptive immune systems. Estimating the immune responses after SARS-CoV-2 vaccinations is one of the important parameters to make predictions of the pandemic and the need for the booster vaccine doses. Therefore, testing the effectiveness of COVID-19 vaccines in different vaccination schedules is necessary, but there are a variety of vaccines and their schedules based on the availability of the vaccines in the global supply. Following primary vaccination, antibody and T cell responses have been decreased over time1. A booster dose six months after the second dose of various vaccines significantly increased neutralizing antibody concentrations2. The heterologous vaccine regimens were reported to stimulate neutralizing antibodies more than the homologous vaccine protocols3.
In Turkey, the inactivated vaccine CoronaVac (Sinovac Life Sciences, Beijing, China) was the first one that was approved by the Ministry of Health. Healthcare workers and individuals over 65 years of age were suggested two doses of CoronaVac administered two months apart in the initial phase of the vaccination program4. After about six months, the Ministry of Health of Turkey recommended a booster of mRNA vaccine (BNT162b2) as an alternative option to the CoronaVac booster. In early studies of the CoronaVac, effectiveness after two-dose schedules was reported as 60-90% 4,5. Nevertheless, these studies were performed approximately 6 weeks after the second dose. After six months of the second dose of CoronaVac, neutralizing antibody titers declined below the seropositivity cut-off but a remarkable increase in the neutralizing antibody concentrations was observed with administration of a third dose 6. However, knowledge of the humoral and cellular immune responses elicited by BNT162b2 and CoronaVac boosters following two doses of primary CoronVac vaccination is still limited. In this study, we aimed to explore the neutralizing antibody and T cell responses after the booster doses of CoronaVac and BNT162b2 following two doses of CoronaVac.