The Efficacy of Sinovac and AstraZeneca COVID-19 Vaccines in Eliciting an Immune Response Against the SARS-CoV-2 Delta Variant in the Department of Medical Sciences Staff, Ministry of Public Health, Thailand

Abstract

Background: Various types of COVID-19 vaccines were approved for emergency use, and all vaccine platforms were initially developed based on the Wuhan-Hu-1 strain. However, with the emergence of the Delta variant pandemic in the middle of 2021, the efficacy of vaccines in the Thai population against the Delta variant was limited. This has raised concerns among people regarding the efficacy and safety of the vaccines.

Objective: To assess the efficacy of the Sinovac and AstraZeneca COVID-19 vaccines in inducing immunity against the SARS-CoV-2 Delta variant.

Method: This experimental study was conducted from May 2021 to July 2022 to investigate the immune response against the SARS-CoV-2 Delta variant, employing the Plaque Reduction Neutralisation Test (PRNT). The protective level was deemed to be reached when the geometric mean (GM) of the PRNT₅₀ value exceeded 10. Antibodies collected from 185 DMSc staff were tested to represent the population under study.

Result: The results revealed that 165 participants who completed two doses of Sinovac and 20 participants who completed two doses of AstraZeneca over one month exhibited geometric mean (GM) PRNT50 values of 19.18 (95% CI = 17.14-21.46) and 62.37 (95% CI = 39.29-99.03), respectively. Subsequently, we examined the PRNT50 values after administering a booster dose to participants who initially received two doses of Sinovac. The findings indicated that 12 participants who received Pfizer-BioNTech and 90 participants who received AstraZeneca achieved PRNT50 values up to 17 times and 6 times, respectively. After a five-month follow-up, the GM of PRNT50, when compared to the third month, showed a slight reduction.

Conclusion: The study findings indicated that the immunity level observed with two doses of Sinovac was lower compared to that achieved with two doses of AstraZeneca. Notably, booster doses administered to the population group that initially received Sinovac showed a higher level of antibodies, demonstrating effective neutralization of the Delta variant for at least 5 months.