Low risk of reinfections and relation with serological response after recovery from the first wave of COVID-19.

The aim of the study was to assess reinfection rates in relation to long-term antibody dynamics against SARS-CoV-2 after the first wave. A prospective longitudinal study 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 COVID-19 attending Udine Hospital (Italy) from March to May 2020. During the follow-up, reinfections were collected. A total of 546 unselected individuals with COVID-19 acquired from March to May 2020 were included (292 female, mean age 53 years). After a median follow-up of 10 months (IQR 6.2-10.4), reinfection occurred in 6 (1.1%) patients, median age of 44.5 years (IQR 33‒49). All had a previous history of mild COVID-19 (all were healthcare workers) and reinfection occurred a median of 9 months (IQR 8.2‒10.2) after the onset of the first episode. Patients with reinfection were either seronegative (2/56, n = 3.6%), seroreverted (2/137, 1.5%), or seropositive (2/353, 0.6%) (p = 0.085). All reinfections were mild (n = 5) or asymptomatic (n = 1). After reinfection, none of patients developed IgM response and only two had a transitory boosted IgG immunization response. In an unselected population after the first wave of COVID-19, after a prolonged observation period (mean 10 months), reinfection was very uncommon; occurred in patients with a previous history of mild infection, mostly with weak or absent serological response; and manifested with mild or asymptomatic clinical presentation.