This study aimed to evaluate the mixed and homogeneous application of the inactivated SARS-CoV-2 vaccine CoronaVac (CV) and the mRNA vaccine BNT162b2 (BNT). This prospective cohort study included 235 health care workers who had received two prime shots with CoronaVac. They were divided into three cohorts after the third month: Cohort-I (CV/CV); Cohort-II (CV/CV/CV); and Cohort-III (CV/CV/BNT). Anti-S-RBD-IgG and total anti-spike/anti-nucleocapsid-IgG antibody concentrations were examined in vaccinated health workers at the 1st, 3rd, and 6th months following the second dose of the vaccination. The mean age of 235 health care workers who participated in the project was 39.51 ± 10.39 (min-max: 22–64). At the end of the 6th month, no antibodies were detected in 16.7% of Cohort-I participants, and anti-S-RDB IgG levels showed a decrease of 60% compared to the levels of the 3rd month. The antibody concentrations of the 6th month were found to have increased by an average of 5.13 times compared to the 3rd-month levels in Cohort-II and 20.4 times in Cohort-III. The heterologous vaccination strategy “CoronaVac and BNT162b2 regimen” is able to induce a stronger humoral immune response and it will help remove inequalities in the developing world where CoronaVac was the initial prime.

This prospective cohort study aimed to evaluate the efficacy of COVID-19 vaccine schemes, homologous versus heterologous vaccine strategies, and vaccine-induced anti-S-RBD-IgG antibody response in preventing COVID-19 among 942 healthcare workers 1 year after vaccination with the inactivated and/or mRNA vaccines. All participants received the first two primary doses of vaccines, 13.6% of them lacked dose 3, 50.5% dose 4, and 90.3% dose 5. Antibody levels increased with the increase in number of vaccine doses and also in heterologous vaccine regimens. In both inactive, mRNA vaccines and mixed vaccination, infection rates were significantly higher in two-dose-receivers, but lower in four- or five-dose receivers and increasing the total number of vaccine doses resulted in more protection against infection: the three-dose regimen yielded 3.67 times more protection, the four-dose 8 times, and five-dose 27.77 times more protection from COVID-19 infection, compared to any two-dose vaccination regimens. Antibody levels at the end of the first year of four- or five-dose-receivers were significantly higher than two- or three-dose receivers. To conclude, an increased number of total vaccine doses and anti-S-RBD antibody levels increased the protection from COVID-19 infection. Therefore, four or more doses are recommended in 1 year for effective protection, especially in risk groups.

Background: For a sustained and essential protective antibody response, it is important to understand how long the humoral immune response induced by the SARS-CoV-2 inactivated vaccine persists. Aims: This study aimed to detect the first and third-month concentrations and seroconversion rates of the antibodies induced by the inactivated vaccine. Study Design: This is a vaccine efficacy study. Methods: The study included 272 health workers who were vaccinated at days 0 and 28 by the inactivated SARS-CoV-2 vaccine (3μg/0.5ml). Anti-S-RBD-IgG and total anti-spike/anti- nucleocapsid-IgG antibody concentrations and seroconversion rates were examined in vaccinated health workers at the 1 ^st and 3 ^rd months after the vaccination. The test method used for the qualitative detection and differentiation of IgG antibodies (indirect method) to SARS-CoV-2 is a chemiluminescence reaction (CLIA). Results: The mean age of the health workers was 38.93±10.59 (min:21-max:64). A total of 45(16.5%) participants declared to have had COVID-19 before the first dose of the inactivated vaccine. The participants were found to be reactive for anti-S-RBD-IgG antibodies by 98.2% and 97.8% at the first and third months, respectively, after the administration of the second dose. The decrease in the mean plasma concentrations of anti-S-RBD IgG was observed as 56.7% in the cohort with only two doses of the vaccine (1 ^st month:42.4AU/ml versus 3 ^rd month: 18.2AU/ml). In the cohort with a history of COVID-19 prior to the vaccination, the decrease was observed as 25.1% (1 ^st month:58.29 versus 3 ^rd month:43.64 AU/ml) and at a mean of 57.4 (0-90) days prior to vaccination, the decrease was of 43.1% (1 ^st month:55.05 AU/ml versus 3 ^rd month:31.28 AU/ml), keeping more stable in participants infected at a mean of 183.1 (91-330) days prior to vaccination (a decrease of 5.2%; with 62.34 AU/ml at 1 ^st and 59.08 AU/ml at 3 ^rd months). Anti-S-RBD concentrations were observed to increase 10-fold (30.44 AU/ml at 1 ^st and 310.64 AU/ml at 3 ^rd months) in participants infected after the vaccination and to decrease among people aged 50 years and older. Conclusion: Antibody concentrations at the 1 ^st and 3 ^rd months after the vaccination with two doses of the inactivated SARS-CoV-2 vaccine were found to be decreased, but still detectable (except in one participant). As participants who had COVID-19 at a mean of 181 (90-330) days before the vaccination presented with a more stable antibody level, it can be concluded that a booster at months 6-12, resulting in a schedule of 0-1-6 months, is recommended for the inactive SARS-CoV-2 vaccination.

This study aimed to evaluate the mixed and homogeneous application of the inactivated SARS-CoV-2 vaccine CoronaVac (CV) and the mRNA vaccine BNT162b2 (BNT). This prospective cohort study included 235 health care workers, who had received two prime shots with CoronaVac. They were divided into three cohorts after the third month: Cohort-I (CV/CV); Cohort-II (CV/CV/CV) and Cohort-III (CV/CV/BNT). Anti-S-RBD-IgG and total an-ti-spike/anti-nucleocapsid-IgG antibody concentrations were examined in vaccinated health workers at the 1st, 3rd and 6th months following the second dose of the vaccination. The mean age of 235 health care workers who participated in the project was 39.51±10.39 (min-max: 22-64). At the end of the 6th month, no antibodies were detected in 16.7% of Cohort-I participants, and anti-S-RDB IgG levels showed a decrease of 60% compared to the levels of the 3rd month. The antibody concentrations of the 6th month were found to have increased by an average of 5.13 times compared to the 3rd-month levels in the Cohort-II and 20.4 times in Cohort-III. The heterologous vaccination strategy “CoronaVac and BNT162b2 regimen” is able to induce a stronger immunity and it will help remove inequalities in the developing world where CoronaVac was the initial prime.

Vaccination induces the development of long-lasting immunity necessary for a sustained and essential protective antibody response. Understanding how long the humoral immune response induced by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) inactivated vaccine lasts is important to create an effective immunization scheme. This study aimed to detect the first, third, sixth month concentrations and seroconversion rates of the antibodies induced by the inactivated vaccine. The study included 282 healthcare workers who were vaccinated at days 0 and 28 by the inactivated SARS-CoV-2 vaccine (3 mu g/0.5ml). Anti-S-RBD-IgG (receptor binding domain) antibody concentrations and seroconversion rates were examined in vaccinated healthcare workers at the first, third and sixth months after the vaccination. MAGLUMI SARS-CoV-2 S-RBD IgG (CLIA) (Snibe Diagnostics, Chinese) kit was used for the detection of antibodies. The mean age of the healthcare workers was 38.93 +/- 10.59 (min=21-max=64). After the administration of the second vaccine dose, the participants were found to be reactive for anti-S-RBD-IgG antibodies by 98.2% and 97.8% at the first and third months, respectively,. The decrease in the mean plasma concentrations of anti-S-RBD IgG was observed as 56.7% in the cohort with only two doses of the vaccine (first month: 42.4 AU/ml versus third month: 18.2 AU/ml). In the cohort with a history of coronavirus disease 2019 (COVID-19) prior to the vaccination, the decrease was observed as 25.1% (first month: 58.29 versus third month: 43.64 AU/ml) and at a mean of 57.4 (0-90) days prior to vaccination, the decrease rate was 43.1% (first month: 55.05 AU/ml versus third month: 31.28 AU/ml), keeping more stable in participants infected at a mean of 183.1 (91-330) days prior to vaccination(a decrease of 5.2%; with 62.34 AU/ml at first and 59.08 AU/ml at third months). At the end of the sixth month, antibodies could not be detected in 16.7% of people who (n=42) received two doses of the inactivated vaccine, and the amount of anti-S-RDB IgG decreased by an average of 52.5% compared to the third month, and by 74.8% compared to the first month. Antibody concentrations at the first, third, and sixth months after the vaccination with two doses of the inactivated SARS-CoV-2 vaccine were found to be decreased and at the sixth month, the rate of non-reactive people was 16.7%. As participants who had COVID-19 at a mean of 181 (90-330) days before the vaccination presented with a more stable antibody level, it can be concluded that a booster at months 6-12, resulting in a schedule of 0-1-6 months, is recommended for the inactive SARS-CoV-2 vaccination for effective herd immunity.