The Fall in Antibody Response to SARS-CoV-2: A Longitudinal Study of Asymptomatic to Critically Ill Patients up to 10 Months after Recovery

The aim of this study was to assess the long-term dynamics and factors associated with the serological response against the severe acute respiratory syndrome coronavirus 2 after primary infection. A prospective longitudinal study was conducted with monthly serological follow-up during the first 4 months, and then at 6, 8, and 10 months after the disease onset of all recovered adult in- and outpatients with coronavirus disease 2019 (COVID-19) attending Udine Hospital (Italy) during the first wave (from March to May 2020). A total of 546 individuals were included (289 female, mean age 53.1 years), mostly with mild COVID-19 (370, 68.3%). Patients were followed for a median of 302 days (interquartile range, 186 to 311). The overall seroconversion rate within 2 months was 32% for IgM and 90% for IgG. Seroreversion was observed in 90% of patients for IgM at 4 months and in 47% for IgG at 10 months. Older age, number of symptoms at acute onset, and severity of acute COVID-19 were all independent predictors of long-term immunity both for IgM (b, linear regression coefficient, 1.10, P = 0.001; b 5.15 P = 0.014; b 43.84 P = 0.021, respectively) and for IgG (b 1.43 P < 0.001; b 10.46 P < 0.001; b 46.79 P < 0.001, respectively), whereas the initial IgG peak was associated only with IgG duration (b 1.12, P < 0.001). IgM antibodies disappeared at 4 months, and IgG antibodies declined in about half of patients 10 months after acute COVID-19. These effects varied depending on the intensity of the initial antibody response, age, and burden of acute COVID-19.