SARS-CoV-2 IgG antibody status in unvaccinated and 2-dose vaccinated Indonesians

 Indonesia began deploying a COVID-19 vaccine in January 2021, prioritising vaccination for high-risk groups such as healthcare workers, the elderly and those with comorbidities, and ending with the general public due to limited vaccine availability. Our study aimed to evaluate antibody response in Indonesians who had received two doses of the vaccine versus those who had not. The study design was a cohort study involving 46 unvaccinated people and 23 people who had received the second dose of the AstraZeneca vaccine over three months ago. Methods used for the qualitative and quantitative detection of IgG antibodies included rapid RI-GHA and ELISA tests. Findings showed that positive IgG antibodies qualitatively detected by the rapid RI-GHA test were significantly higher in those vaccinated (60.9%) than in unvaccinated people (26.1%). Using the ELISA assay, all vaccinated individuals qualitatively showed positive antibodies (cut-off ≥4.33 BAU/mL), and the average quantitative titer of anti-SARS-CoV-2 s-RBD IgG was significantly higher in vaccinated (157.06±238.68 BAU/mL) than in unvaccinated (51.90 ± 87.60 BAU/ml) individuals. Some unvaccinated individuals had anti-SARS-CoV-2 antibodies, which could be due to asymptomatic or symptomatic infection without history of COVID-19 realising, although their mean antibody titers were certainly lower than those in the 2-dose vaccinated group. Approximately 56% of vaccinated individuals had antibody titers above 60 BAU/mL (as a cut-off for protective threshold), a significantly higher proportion than unvaccinated individuals. In conclusion, vaccination with two doses increased anti-SARS-CoV-2 antibodies which resulted in enhanced immunity against symptomatic COVID-19.