IgG antibody response to SARS-CoV-2 among recovered and vaccinated people in Georgia

Revaz Metchurtchlishvili, Nikoloz Chkhartishvili, Akaki Abutidze, Tengiz Tsertsvadze


Background: Coronavirus Disease 2019 (COVID-19) first identified in China in December 2019, rapidly spread and reached pandemic level by March 2020. Achieving population level immunity against Severe Acute Respiratory Syndrome Coronavirus 2 (SAR-CoV-2) has been considered the main direction to ending the pandemic. Better characterization of immune response to SARS-CoV-2 is essential for effectively tackling the virus through informing prevention, treatment and control strategies.

Aim: The objectives of this study were to longitudinally evaluate IgG antibody response among recovered COVID-19 patients and vaccinated persons and second to assess protective efficacy by estimating risk of re-infection and breakthrough infection.

Methods: We evaluated two cohorts of adults: 1) recovered COVID-19 patients without vaccination (followed for the median 23 weeks) and 2) vaccinated persons without the history of COVID-19 infection (followed for the median 5 weeks). On each follow up visit blood was drawn for IgG antibody testing and participants were interviewed about the potential exposure to SARS-CoV-2 and diagnosis of infection.

Results: Study enrolled 250 persons, including 150 recovered from COVID-19 and 100 vaccinated against COVID-19. Among recovered 150 COVD-19 persons SARS-CoV-2 IgG antibody seropositivity decreased from 95.3% to 92.0% during the median 23 weeks of follow-up after diagnosis. Among vaccinated 100 persons 93 (93.0%) were IgG seropositive after the median 5 weeks of follow-up since the first dose of vaccine. Among vaccinated persons followed for at least 2 weeks, the rate of seropositivity was 96.8% (92/95). Kaplan-Meier analysis showed that among recovered persons the estimated probability of having SARS-CoV-2 IgG antibodies gradually declined over time dropping to 0.89 by week 36 after diagnosis. Among vaccinated persons, the probability of having SARS-CoV-2 IgG antibodies steeply increases over time reaching 0.99 by week 12 after vaccination. During the follow-up none of recovered patients had re-infection (incidence: 0.0 per 100 person-weeks, 95% CI: 0.0-0.1). No cases of breakthrough infections were reported among vaccinated persons (incidence: 0.0 per 100 person-weeks, 95% CI: 0.0-0.6).

Conclusions: Our study confirms that both natural infection and vaccination generate robust antibody response to SARS-CoV-2. There were no new infections diagnosed during the follow-up, corroborating previous findings of high protective efficacy of this response within the 6 months post-infection or vaccination(1). Data also indicates that SARS-CoV-2 specific IgG antibodies among recovered patients wane over time. Future studies to evaluate comprehensive immune response to the virus over extended periods of time, including dynamics of humoral and cellular immunity, are warranted to better inform prevention, treatment and control strategies.


COVID-19; Immunity; Natural infection; Vaccination; Immunoglobulin G.

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ISSN: 2346-8491 (online)