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Aims: Limited information is available about the efficacy and adverse events of COVID-19 vaccines introduced into public use with Emergency Usage Licences without completing phase-3 trials. Data refer to healthy and mostly younger people, with lacking evidence about the protectivity. This study aimed determining seroconversion rates and levels specific to anti-S-RBD IgG and total anti-spike/anti-nucleocapsid IgM and IgG antibodies against SARS-CoV-2 after the administration of two doses of the inactivated SARS-CoV-2 vaccine in Turkey, comparing three types of antibodies. It was also aimed to assess short-term adverse events due to the vaccine. It is intended to answer the questions about efficacy and safety, which lack in phase trials, especially at community level. It is also aimed to collect data, which will form the basis for assessing whether antibodies are protective at different community settings. Methods: The study carried out in Turkey, comprised 282 healthcare workers who received two doses of the inactivated SARS-CoV-2 vaccine administered in two 3µg doses, 28 days apart. In day-28 after the second dose, anti-S-RBD IgG and total anti-spike/anti-nucleocapsid IgM and IgG antibodies against SARS-CoV-2 were detected by in-vitro chemiluminescence immunoassay. Results: At day 28 after the second dose, the seroconversion rates were found to be 92.9% for total anti-spike/anti-nucleocapsid IgG and 15.2% for IgM and 98.2% for anti-S-RBD IgG antibodies. The immunogenicity was affected by pre-vaccination natural COVID-19 history and age. The incidence of at least one adverse event was found as 29.8% after the first dose and 24.1% after the second dose, with the most common events being pain at the injection site, weakness, fatigue and headache. Conclusion: A high rate of seroconversion was observed with no serious adverse events. Prior natural COVID-19 history and age significantly contributed to strong immunogenicity. A booster dose appears required for older ages and individuals without immune response.

Background/aim Various COVID-19 vaccines are being developed around the world. Important questions to be answered regarding vaccines are efficacy, safety, and whether antibodies are protective when used in different communities. This study aimed to determine seroconversion rates of the inactivated SARS-CoV-2 vaccine in healthcare workers in a hospital and short-term adverse events due to the vaccine. Materials and methods The study carried out in Cukurova University, Turkey, comprised 282 healthcare workers who received two doses of the inactivated SARS-CoV-2 vaccine administered in two 3 µg doses, 28 days apart. On day 28 after the second dose, anti-S-RBD IgG and total anti-spike and anti-nucleocapsid IgM and IgG antibodies against SARS-CoV-2 were detected by using in vitro chemiluminescence immunoassay method. Results The mean age of participants was 39.06±10.65 (min 21; max 65) with 43.6% males and 56.4% females. On day 28 after the second dose, the seroconversion rates were found to be 92.9% for total anti-spike and anti-nucleocapsid IgG and 15.2% for IgM and 98.2% for anti-S-RBD IgG antibodies and having natural COVID-19 prior to vaccination, age and comorbidity were found to be significant factors for immunogenicity. The incidence of at least one adverse event was found as 29.8% after the first dose and 24.1% after the second dose, with the most common events being pain at the injection site, weakness, fatigue and headache. Conclusion On day 28 after the second dose of 3 ?g of the inactivated SARS-CoV-2 vaccine administration, a high rate of seroconversion was observed with no serious adverse event. Prior natural COVID-19 history, age and comorbidity were significant contributors to the formation of a strong immune response. It can be concluded that a third dose may be considered in people aged 50 years and older and those with comorbidities.