Follow-up of IG G antibody response in COVID-19 cases

Introduction: In December 2019, pneumonia cases of unknown origin were reported from the city of Wuhan, China. The virus isolated from respiratory tract samples taken from these cases was named as SARS-CoV-2. It has caused a pandemic that spread all over the world due to its transmission from person to person through droplets. It is known that IgM and IgG antibody responses occur in people who have had Covid-19. However, it is not known whether this antibody response against SARS-CoV-2 will be permanent. In our study, we aimed to determine whether the antibody response against SARS-CoV-2 would be permanent by following the antibody levels quantitatively for about a year in cases with Covid-19, and which variables affected the antibody titer level and the duration of antibody positivity. Materials and Methods: Antibody titer follow-up was planned for one year in cases who applied to Bezmialem Vakıf University Medical Faculty Hospital between March 11, 2020 and August 11, 2020 and were diagnosed with Covid-19. Comorbid chronic diseases of the patients participating in the study were recorded, Charlson comorbidity index (CCI) and body mass index (BMI) were calculated. According to the Covid-19 treatment guideline of the American National Institutes of Health, the clinical severity scores of the patients were determined and the patient population was divided into 3 clinical severity groups as mild, moderate and severe. The patients were called for a total of 4 visits approximately in the 1st, 3rd, 6th and 12th months from the first admission to the hospital, and the SARS-CoV-2 Anti-N IgG titers were measured with SARS-CoV-2 IgG Quant test (Abbott Laboratories, IL, USA). Results: 53.1% of the 177 patients included in the study were male (n=94) and their mean age was 44.72±13.47. 164 (92.7%) of the patients were PCR positive, 13 cases were diagnosed with probable Covid-19. SARS-CoV-2 Anti-N IgG test was positive in all cases with a possible diagnosis of Covid-19. The median BMI value of our patients was 27.74 (IQR: 24.97-30.89). Seventy patients (39.5%) had at least one comorbidity. According to the severity, 33.3% of the patients were classified as mild, 46.9% as moderate and 19.8% as severe. Day intervals of follow-up visits; Median 56 days at visit 1 (IQR: 42-64 days), median 92 days at visit 2 (IQR: 90-96 days), median 193 days at visit 3 (IQR: 183-199 days), 371 days at visit 4 (IQR: 366-405 days). The seroconversion rate was 88.7% at visit 1, 80.8% at visit 2, 45.9% at visit 3, and 21.4% at visit 4. The median of SARS-CoV-2 Anti-N IgG titers was 5.9 at visit 1 (IQR: 3.45-7.66), 3.88 at visit 2 (IQR: 2.08-6.44), at visit 3 1.36 (IQR: 0.58-2.83), 0.84 (IQR: 0.38-1.22) at visit 4. It was observed that the antibody titers decreased significantly especially at the 3rd visit (p<0.0001). There was a statistically significant correlation between the anti-N IgG titers and the severity of the disease (p<0.0001, p<0.0001, p<0.0001, p=0.004) for all visits, respectively. In multivariate logistic regression analysis, as the disease severity increased, the probability of antibody positivity increased approximately 3 to 6 times at visits 1 and 3 (OR: 6,453 95% CI 2,126-19,589, p<0,0001, OR: 3,169 95% CI 1,710-, respectively). 5,873, p<0.0001). Other independent variables affecting antibody positivity were age and CCI score (OR: 1.078 95% CI 1.031-1.127, p<0.0001, OR: 0.537 95% CI 0.363-0.795, p=0.002). In addition, the vaccinated patient group between the 3rd and 4th visits was compared with the unvaccinated patients. The mean antibody titer in the vaccinated group at the last visit was found to be significantly higher than the unvaccinated group (p<0,0001). Conclusion: In our study, it was observed that there was a significant decrease in Anti-N Ig-G antibody positivity, especially after the 6th month, and a significant decrease was observed in the anti-N IgG titer within 1 year. The mean antibody titers measured at all visits were found to be significantly lower in mild patients than in moderate and severe patients. The antibody titer has been shown to be moderately correlated with the severity score. The most important independent factor affecting antibody positivity was the severity of the disease. Other independent variables affecting antibody positivity are age and CCI score. In our study, antibody titer values were followed, but the protection of these values from re-infection and severe disease could not be evaluated objectively. Further studies are needed to elucidate the relationship between seropositivity and protective immunity against re-infection, as well as the persistence period of antibodies against SARS-CoV-2, and to determine protective antibody titer levels.