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.