ABSTRACT
Introduction: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the first pandemic caused by a coronavirus. Respiratory/aerial droplets transmission and the high number of "hidden" asymptomatic cases play a critical role in the rapid spread of the virus across countries. The seroprevalence of SARS-CoV-2 antibodies in the general population is currently unknown. It has been estimated that undocumented infections were the source for #80% of the documented cases before traveling restriction policies took place. Serological evaluation is essential for investigating the extent of SARS-CoV-2. Even more, assessing the prevalence of anti-SARSCoV-2 in hospital staff offers a unique opportunity to study the correlation between seroconversion and immunization because of their occupational exposure and at higher risk of contagious. Methods: The study enrolled a total of 3242 employees of our hospital, "Policlinico Riuniti" of Foggia. The employees' group was stratified in 3 subgroups according to their relative exposure to SARS-CoV-2 (high, intermediate, and low-risk groups). We used a chemiluminescent immunoassay (CLIA, Shenzhen YHLO Biotech) to study the seroprevalence of SARS-COV-2 specific antibodies (IgG and IgM against nucleocapsid and spike proteins). The cut-off was set to 8 AU/mL for both IgG and IgM (specificity of 98,8% and 100%, respectively). A control group of 83 samples sera collected before the Italian COVID-19 outbreak (2018-19) was also tested. Healthcare workers with IgG or IgM concentration above 6.0 and below 8.0 AU/mL were considered borderline. Nevertheless, all of them were tested for the SARS-CoV-2 viral RNA presence (Allplex™ 2019 n-CoV Assay, Seegene). Results: Sixty-two individuals (1.9%, 1.4-2.3%, 95% CI) tested positive for at least one antibody anti-SARS-CoV-2. Five individuals (8.0% of the positive) had IgG and IgM positive test results, while 32 and 25 had only IgG and only IgM positive results. Instead, viral RNA was detected in only nine individuals (13.8% of Ig positive) by RTPCR. The cumulative proportion of individuals who tested positive (IgG and/or IgM) varied between 1-2.4%. The seroprevalence was lower in the high-risk group 1.4% (6/428,0.5-2.6%, 95% CI) vs. intermediate-risk group 2.0 % (55/2736, 1.5-2.5%, 95% CI). Only one participant (1.3%, 0-3.8%, 95% CI) of the low-risk group tested positive for SARS-CoV-2 IgM antibodies. Conclusions: The low level of seropositivity (1.9%) shows that the COVID-19 containment measures adopted were adequate and effective. Moreover, the combination of both serological and molecular tests can improve the likelihood of identifying asymptomatic subjects.