Seroprevalence of anti-SARS-CoV-2 IgG among healthcare workers of a large university hospital in Milan, Lombardy, Italy: a cross-sectional study

Abstract

Objectives: To assess the seroprevalence of anti-SARS-CoV-2 IgG among health careworkers (HCWs) in our university hospital and verify the risk of acquiring the infection according to work area.

Design: Cross-sectional study.

Setting: Monocentric, Italian, third-level university hospital.

Participants: All the employees of the hospital on a voluntary base, for a total of 4055 participants among 4572 HCWs (88.7%).

Primary and secondary outcome measures: Number of anti-SARS-CoV-2 positive serology according to working area. Association of anti-SARS-CoV-2 positive serology to selected variables (age, gender, country of origin, body mass index, smoking, symptoms and contact with confirmed cases).

Results: From 27 April 2020 to 12 June 2020, 4055 HCWs were tested and 309 (7.6%) had a serological positive test. No relevant difference was found between men and women (8.3% vs 7.3%, p=0.3), whereas a higher prevalence was observed among foreign-born workers (27/186, 14.5%, p<0.001), employees younger than 30 (64/668, 9.6%, p=0.02) or older than 60 years (38/383, 9.9%, p=0.02) and among healthcare assistants (40/320, 12.5%, p=0.06). Working as frontline HCWs was not associated with an increased frequency of positive serology (p=0.42). A positive association was found with presence and number of symptoms (p<0.001). The symptoms most frequently associated with a positive serology were taste and smell alterations (OR 4.62, 95% CI: 2.99 to 7.15) and fever (OR 4.37, 95% CI: 3.11 to 6.13). No symptoms were reported in 84/309 (27.2%) HCWs with positive IgG levels. Declared exposure to a suspected/confirmed case was more frequently associated (p<0.001) with positive serology when the contact was a family member (19/94, 20.2%) than a patient or colleague (78/888, 8.8%).

Conclusions: SARS-CoV-2 infection occurred undetected in a large fraction of HCWs and it was not associated with working in COVID-19 frontline areas. Beyond the hospital setting, exposure within the community represents an additional source of infection for HCWs.

Keywords: diagnostic microbiology; occupational & industrial medicine; public health; virology.

Figure 1

Number of IgG tests (left panel) and percentage of positive IgG tests (right panel) in 162 subjects with a positive nasopharyngeal swab (NPS) prior to serological testing, according to days elapsed since day of first positive NPS. Left panel shows histogram and kernel density smoothing line. In right panel circles indicate subjects with negative (lower circles, n=41) or positive (upper circles, n=121) anti-SARS-CoV-2 IgG, solid and dashed lines are the predicted percentages calculated with a logistic regression model, and dashed lines are 95% bands around the predicted.