key: cord-0962755-1sria1gh
authors: Vimercati, Luigi; Stefanizzi, Pasquale; De Maria, Luigi; Caputi, Antonio; Cavone, Domenica; Quarato, Marco; Gesualdo, Loreto; Lopalco, Pier Luigi; Migliore, Giovanni; Sponselli, Stefania; Graziano, Giusi; Vittoria Larocca, Angela Maria; Tafuri, Silvio
title: Large-scale IgM and IgG SARS-CoV-2 serological screening among healthcare workers with a low infection prevalence based on nasopharyngeal swab tests in an Italian university hospital: perspectives for public health
date: 2021-01-27
journal: Environ Res
DOI: 10.1016/j.envres.2021.110793
sha: d32f34c7ac68265b5213e418cb730cf7615eace6
doc_id: 962755
cord_uid: 1sria1gh

BACKGROUND: Healthcare workers (HCWs) are highly exposed to SARS-CoV-2infection given their specific tasks. The IgG-IgM serological assay has demonstrated good accuracy in early detection in symptomatic patients, but its role in the diagnosis of asymptomatic patients is uncertain. The aim of our study was to assess IgM and IgG prevalence in sera in a large cohort of HCWs previously subjected to Nasopharyngeal swab test (NST) after accurate risk assessment due to positive COVID-19 patient exposure during an observation period of 90 days. METHODS: 2407 asymptomatic HCWs that had close contact with COVID-19 patients in the period between April 8th and June 7(th) were screened with NST based on the RT-PCR method. In parallel, they underwent large-scale chemiluminescence immunoassays involving IgM-IgG serological screening to determine actual viral spread in the same cohort. RESULTS: During the 90-day observation period, 18 workers (0.75%) resulted positive for SARS-CoV-2 infection at the NST, whereas the positivity rates for IgM and IgG were 11.51% and 2.37%, respectively (277 workers). Despite high specificity, serological tests were inadequate for detecting SARS-CoV-2 infection in patients with previous positive NST results (IgM and IgG sensitivities of 27.78% and 50.00%, respectively). CONCLUSIONS: These findings indicate a widespread low viral load of SARS-CoV-2 among hospital workers. However, serological screening showed very low sensitivity with respect to NST in identifying infected workers, and negative IgG and IgM results should not exclude the diagnosis of COVID-19. IgG-IgM chemiluminescence immunoassays could increase the diagnosis of COVID-19 only in association with NST, and this association is considered helpful for decision-making regarding returning to work.

BACKGROUND SARS-CoV-2 is a huge challenge for healthcare workers worldwide. The specific tasks of healthcare workers include daily contact with infected people, and the Hospital Health Administration is forced to rapidly adapt work conditions to avoid nosocomial cluster [1] . However, after the first large European wave of infection between March and May, the most recent literature focuses attention on asymptomatic patients as an effective and efficient source of contagion [2] ; the ability to intercept these patients is crucial to avoid new clusters and lockdown measures.

To date, among all available diagnostic methods for detecting SARS-CoV-2, real-time reverse transcription polymerase chain reaction (RT-PCR) using respiratory samples is the gold standard for COVID-19 diagnosis, but the combination of IgM and IgG antibodies offer increased sensitivity [3] . Moreover, Deeks et al. affirmed that IgM antibody detection is a sensitive and specific tool to diagnose recent SARS-CoV-2 infection at least 15 days after close contact with an infected individual if NST was negative [4] .

To date, automated chemiluminescent immunoassay (CLIA) is the most validated serological test and seems to increase RT-PCR sensitivity [5] . Recently, high sensitivity rates were described in IgM and IgG CLIA determination (88% and 100% after 12 days of symptom onset) [6, 7] . On the other hand, rapid detection SARS-CoV-2 antibody tests, e.g., lateral flow immunoassays (LFIAs), seem to have lower accuracy [8, 9] ; in particular, the immunochromatographic antibody test is burdened by the high incidence of false positive results of IgG [10] .

The longitudinal profile of IgM and IgG kinetics revealed seroconversion for both within 6 days with pike times of 18 and 23 days, respectively [11] . A positive IgG and/or IgM result in a single sample collected 2 weeks after symptoms in patients who were negative based on NST suggests SARS-CoV-2 infection; however, today, minimal evidence is available for the asymptomatic population [12] .

The aim of this study was to assess IgM and IgG prevalence in sera in a large cohort of HCWs previously subjected to NST after accurate risk assessment due to positive COVID-19 patient exposure during an observation period of 90 days.

Study group. All HCWs of the University Hospital of Bari, Italy underwent a preventive protocol that required them to undergo a NST in case of close contact with COVID-19 patients or evidence of SARS-CoV-2 symptoms onset (anosmia, ageusia, fever, asthenia, sore throat, rhinorrhea, cough, diarrhea, and dyspnea). All HCWs subject to NST, after 14-21 days, underwent sera collection for SARS-CoV-2 IgM and IgG determination. Occupational risk assessment was performed according to Italian Guidelines for Biological Occupational Risk and CDC guidance [13, 14] . The three risk categories were high, medium, and low.

Specimen collection and analysis. All the selected workers were submitted to a collection of nasopharyngeal SWAB specimens by trained staff following adequate standard operating procedures (SOPs), and during the collection, all specimens were handled carefully according to World Health Organization (WHO) criteria [15] [16] [17] . The specimens underwent Nucleic Acid Amplification Tests (NAAT) for COVID-19. This method is based on the detection of unique The risk, both in univariate and multiple regressions, was expressed as an odds ratio (OR) with the relative 95% confidence interval (CI). All the results with a p-value<0.05 were considered statistically significant. The diagnostic accuracy of serological tests compared with the SWAB test was also evaluated. The measures considered included sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV). All analyses were conducted with SAS software (version 9.4).

During the 90-day observation period, 2407 of 5750 HCWs (41.86%) referred to at least one close contact with COVID-19 patients and underwent NST and serological determination in a period between the 14 th and 21 st days after J o u r n a l P r e -p r o o f NST. Table 1 shows all sociodemographic, professional and clinical characteristics of the HCWs enrolled. Participants were middle-aged (45.67±11.96) and predominantly women (56.5%). A large part of the sample (approximately 70%) was composed of physicians and nurses (60.85%) mostly exposed, during their daily practice, to a medium biological risk, i.e. HCWs that provide direct assistance to patients, in the absence of procedures that generate aerosols, in the patient room / ward.The absolute prevalence of HCWs with a nasopharyngeal SWAB test positive for SARS-COV-2 infection was 18 out of 2470 (0.75%). The prevalence of a positive IgM serological test was 277 HCWs out of 2470 (11.51%), and the prevalence for IgG was 57 out of 2470 (2.37%). Table 2 shows the association between the HCWs and the three main outcomes of the survey. The only determinant associated with the positive SWAB test was job title (p-value=0.027); subjects with SARS-COV-2 infection were most frequently physicians (72.2%).

The IgM test results were associated with age (p-value=0.0004), biological risk (p-value=0.0004), job title (p-value=0.0187) and operative units (p-value≤0.0001). In particular, seropositive workers were younger (43.3±12.2), were more exposed to medium (67.0%) biological risk and were more frequently physicians (40.8%) working in clinical operative units (66.5%) compared with workers testing negative. The IgG test results were not associated with any of the determinants considered. Four HCWs remained asymptomatic until healing. The contact tracing protocol revealed that 17 of 18 HCWs had been in close contact with COVID-19 cases in non-workplace settings (households, schools, group-living and other social environments); only one, a 56-year-old physician, was in close contact with a COVID-19 patient at work.Univariate analyses were performed to express the previous associations in terms of odds ratios with 95% confidence intervals. The results are shown in Table 3 After double screening, we found an unexpectedly high serological prevalence of SARS-CoV-2 infection despite a low rate of RT-PCR positivity. IgM positivity was higher in each HCW category (nurse, physician, SHOs) with respect to IgG prevalence.

Considering the high specificity of the chemiluminescence immunoassay found in previous studies [6] and calculated in our work using NST as the gold standard, these results could indicate the existence of numerous undiagnosed COVID-19 cases among HCWs in the assistant setting; infected people likely remained asymptomatic, which could explain the trouble in identifying these workers through NST [20] . Moreover, the large difference in positive results between serological and nasopharyngeal tests could indicate many sources of unknown exposure to SARS-CoV-2 despite the implementation of preventive measures. An Italian cross-sectional study highlighted that approximately half of infected HCWs had no significant personal history of SARS-CoV-2 exposure, suggesting that many sources of contagion were unapparent. Thus, the exclusive use of RT-PCR screening is complex and not very useful [21] . Moreover, an appropriate window period seems to be essential to enhance the sensitivity of serological tests, increasing the gap in the positive result rate with respect to RT-PCR. In a multicenter retrospective study in Wuhan, China, 47% of SARS-CoV-2-infected people, who were mostly asymptomatic, were diagnosed with serological tests after negative RT-PCR [22] , as demonstrated by Clarke and colleagues. A high seroprevalence of SARS-CoV-2 antibodies was noted in asymptomatic or PCR-negative patients receiving in-center hemodialysis, suggesting that current diagnostic screening strategies may be limited in their ability to detect acute infection [23] . In our study, we found a high seroprevalence of SARS-CoV-2 with the Abbott test, but low sensitivity in identifying virus carriers and low positive predictive values in 2407 serum samples. Thus, serological tests did not adequately indicate SARS-CoV-2 infection in patients with a previous positive NST result (IgM and IgG sensitivity of 27.78% and 50.00%, respectively). These results are in contrast to those from Batra and colleagues [24] , who found that the test had a specificity of 100% and sensitivity of 99.1% for specimens collected >14 days post symptom onset or >5 days post-RNA testing. On the other hand, Meschi et al. [25] showed that the Abbott system could have lower sensitivity, which is more similar to our results (61.9% on the fourteenth day).

These results suggest that NST based on the RT-PCR method should be the gold standard for guiding decisions on quarantine and readmission to work in occupational health surveillance.

At present, serological tests cannot replace the molecular diagnostic test as they are not characterized by sufficient validity due to their low sensitivity in identifying virus carriers. Therefore, there are no recommendations for their use for both diagnostic (early identification of infected subjects) and prognostic purposes in occupational settings, nor to determine the work suitability for each worker. J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f 

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