Evaluation of antibody response after two doses of the Sinovac vaccine and the potential need for booster doses in cancer patients.

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is more severe in some specific patient groups, such as cancer patients with a mortality rate of 26.5%. The main way of protection is vaccination. In this study, we aimed to evaluate the antibody responses of our cancer patients who received two doses of the inactivated COVID-19 vaccine manufactured by Sinovac Life Sciences. Patients over the age of 18, who had not been suspected or polymerase chain reaction-confirmed COVID-19 positive, who received two doses of the vaccine, and at least 28 days passed after the second dose, and who received at least one dose of cancer treatment before the vaccination-were included in the study. Immunoglobulin class G antibodies against the receptor binding region (S-RBD) of the spike protein S1 subunit of SARS-CoV-2 were studied. A total of 200 patients with a diagnosis of cancer were included in the study. The median time between the second dose of the Sinovac vaccine and the time of blood collection was 3.44 (3.20-3.84) months. SARS-CoV-2 antibody positivity was detected in 110 (55%) patients. The two subgroups with the highest antibody levels were gynecological cancers and breast cancers, with median 158.5 AU/ml (38.4-764.5) and 106.3 AU/ml (61.9-162.9) levels, respectively. Antibody positivity rate was 46.8% in patients who received chemotherapy at any time between the first dose of the vaccine and the date of blood collection; and it was 73.8% in the group that did not receive chemotherapy (p < 0.001). As a result, the expected antibody response was not obtained with two doses of the Sinovac vaccine. Therefore, if the Sinovac vaccine is to be preferred for these patients, the appropriate booster time for the third dose should be determined or other vaccines, such as messenger RNA vaccines, with reported higher antibody responses should be considered.