Preliminary assessment of COVID-19 serological situation in a high-risk cohort

Abstract

Abstract Introduction Studies on the duration of immune response in subjects with COVID-19 could facilitate understanding of how immunity develops and persists during the natural course of infection, as well as providing epidemiological information useful in public health decisions on the use of vaccines. The study aims to analyse antibody response in a cohort of subjects with high-risk of exposure to SARS-CoV-2 due to the long-term care facility environment. Methods We recruited 657 subjects between guests and healthcare workers from Piedmontese nursing homes. IgG directed against the S1 domain of SARS-CoV-2 spike protein were assayed on serum. We used EUROIMMUN kit, which exploits ELISA method. Reference values are (RU/mL): positive ≥ 11; borderline 8 ≤ value < 11; negative < 8. Results Among the 657 subjects, we examined 494 health workers and 163 guests. The average age was 56.2. In relation to COVID-19, 402 subjects had a previous infection, 255 had not. About serology: positive 369; negative 261; borderline 27. The mean of the IgG values was 35.68 RU/mL. Swabs/serology: 77.36% with previous infection were positive to serology, 18.66% negative; of all subjects who were always negative to swabs, 72.94% were negative, 22.74% positive. Moreover, the mean time elapsed from positive swab to serology test was 144.08 days. Conclusions Preliminary results show that the antibody response is present and persistent in the study population, even a long time after the infection. According to literature, antibody persistence has great variability in our population, as no statistically significant correlations were found between antibody titer and other variables such as time elapsed since infection. The examined sample, belonging to high-risk category (long-term care facilities), is most likely to present repeated exposure to SARS-CoV-2, highlighted by the seropositivity of never infected subjects. These findings might help policy makers better address resources regarding vaccines. Key messages Antibody response is present and persistent in the study population, even a long time after the infection. Antibody persistence has great variability in our population, as no statistically significant correlations were found between antibody titer and other variables such as time elapsed since infection.