Anti-SARS-CoV-2 IgG Antibody Levels Among Thai Healthcare Providers Receiving Homologous and Heterologous COVID-19 Vaccination Regimens

Background: We examined SARS-CoV-2 anti-spike 1 IgG antibody levels following COVID-19 vaccination (AstraZeneca [AZ], Sinovac [SV], Pfizer-BioNTech [PZ]) among Thai healthcare providers. Methods: Blood specimens were tested using enzyme-linked immunosorbent assay. We analyzed seven vaccination regimens: a) one dose of AZ or SV, b) two doses of homologous (2AZ, 2SV) or heterologous (1AZ+1PZ) vaccines, and c) three doses of heterologous vaccines (2SV+1AZ, 2SV+1PZ). Differences in antibody levels were assessed using Kruskal-Wallis statistic, Mann-Whitney test, or Wilcoxon matched-pairs signed-rank test. Antibody kinetics were predicted using fractional polynomial regression. Findings: The 563 participants had median age of 39 years; 92% were female; 74% reported no underlying medical condition. Antibody levels peaked at 22-23 days in both 1AZ and 2SV vaccinees and dropped below assay’s cutoff for positive (35·2 binding antibody units/mL [BAU/mL]) in 55 days among 1AZ vaccinees compared to 117 days among 2SV vaccinees. 1AZ+1PZ vaccination regimen was highly immunogenic (median 2,279 BAU/mL) 1-4 weeks post vaccination. 2SV+1PZ vaccinees had significantly higher antibody levels than 2SV+1AZ vaccinees four weeks post vaccination (3,423 vs. 2,105 BAU/mL; p-value<0·01), and during weeks 5-8 (3,656 vs. 1,072 BAU/mL; p-value<0·01). Antibodies peaked at 12-15 days in both 2SV+1PZ and 2SV+1AZ vaccinees, but those of 2SV+1AZ declined more rapidly and dropped below assay’s cutoff in 228 days while those of 2SV+1PZ remained detectable. Interpretation: 1AZ+1PZ, 2SV+1AZ, and 2SV+1PZ vaccinees had substantial IgG levels, suggesting that these individuals likely mounted sufficient anti-S1 IgG antibodies for possible protection against SARS-CoV-2 infection. Funding Information: U.S. Centers for Disease Control and Prevention.Declaration of Interests: All authors have no conflicts of interests or funding to disclose.Ethics Approval Statement: This study was approved by the Institutional Review Boards (IRBs) of PMK; BIDI; PH; RY; Department of Disease Control of the Thai MOPH (Thailand); and Walter Reed Army Institute of Research (USA). The IRB of the U.S. Centers for Disease Control and Prevention (USA) and Mahidol University (Thailand) relied on the determinations of PMK’s and MOPH’s IRBs, respectively. All participants provided written informed consent.