Persistence of IgG COVID-19 antibodies: A longitudinal analysis

Abstract

Background and aim: The kinetics of antibody production in response to coronavirus disease 2019 (COVID-19) infection is not well-defined yet. This study aimed to evaluate the antibody responses to SARS-CoV-2 and its dynamics during 9-months in a cohort of patients infected during the first phase of the pandemic. As a secondary aim, it was intended to evaluate the factors associated with different concentrations of IgG antibodies.

Methods: A prospective cohort study was conducted from June 2020 to January 2021. This study recruited a convenience sample of adult individuals who where recently diagnosed with COVID-19 and were living in mainland Portugal. A total of 1,695 blood samples were collected from 585 recovered COVID-19 patients up to 9 months after SARS-CoV-2 acute infection. A blood sample was collected at baseline and three, 6 and 9 months after SARS-CoV-2 acute infection to assess the concentration of IgG antibody against SARS-CoV-2.

Results: The positivity rate of IgG reached 77.7% in the first 3 months after symptom onset. The IgG persists at all subsequent follow-up time-points, which was 87.7 and 89.2% in the 6th and 9th months after symptom onset, respectively. Three distinct kinetics of antibody response were found within the 9 months after infection. Kinetic 1 (K1) was characterized by a constant low IgG antibody concentration kinetic (group size: 65.2%); kinetic 2 (K2), composed by constant moderate IgG kinetic (group size: 27.5%) and kinetic 3 (K3) characterized by higher IgG kinetic (group size: 7.3%). People with ≥56 years old (OR: 3.33; CI 95%: [1.64; 6.67]; p-value: 0.001) and symptomatic COVID-19 (OR: 2.08; CI 95%: [1.08; 4.00]; p-value: 0.031) had higher odds of a "Moderate IgG kinetic." No significant association were found regarding the "Higher IgG kinetic."

Conclusion: Our results demonstrate a lasting anti-spike (anti-S) IgG antibody response at least 9 months after infection in the majority of patients with COVID-19. Younger participants with asymptomatic disease have lower IgG antibody positivity and possibly more susceptible to reinfection. This information contributes to expanding knowledge of SARS-CoV-2 immune response and has direct implications in the adoption of preventive strategies and public health policies.

Keywords: COVID-19; IgG; SARS-CoV-2; antibody responses; humoral immune response; post-infection immunity.

Figure 1

Flowchart of the study.

Figure 2

Antibody levels at each considered time-point. (A) total, (B) stratified by gender, (C) stratified by age-group; (D) stratified by disease severity. (A) The median IgG antibody level was 42.8 (IQR, 17.7–87.8) 3 months after symptom onset, 52.4 (IQR, 25.4–93.0) and 54.9 (IQR, 25.4–98.9) in the 6th and 9th months after symptom onset. (B) At the first time-point (three months), the IgG antibody levels were significantly higher in men compared to women (p = 0.003); for the remaining time-points, six (p = 0.776) and nine (p = 0.232) months, no significant differences were found. (C) The IgG antibody levels were significantly higher in the age group 56 years in the three time periods considered, compared to the age groups 18–35 (p ≤ 0.001; p ≤ 0.001; p = 0.005) and 36–55 (p ≤ 0.001; p ≤ 0.001; p = 0.005). (D) The IgG antibody levels were significantly higher in the critical severity level in the three time periods considered, compared to the asymptomatic, mild to moderate (p = 0.003; p ≤ 0.001; p = 0.005) and severe levels (p = 0.020; p = 0.008; p = 0.014).

Figure 3

Antibody kinetics, and proportion of individuals in the groups. Shapes represent observed membership and lines represent predicted group memberships.