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Risk for SARS-CoV-2 Infection in Healthcare Workers, Turin, Italy

Our take —

Among a non-randomly selected group of 5,444 healthcare workers in Torino, Italy, 6.9% had evidence of IgG antibodies from April to May 2020. The seroprevalence was higher among those with close patient contact compared to those without (7.5% vs 5.4%). Older healthcare workers and those with a previous diagnosis of COVID-19 had higher antibody titers, supporting the conclusion that symptomatic infection is associated with increased immune response. Only 46.7% of participants with IgG antibodies had a previous diagnosis, demonstrating that antibody testing can improve our understanding of frequency of infection. However, 17% of those who were previously diagnosed had no evidence of antibodies, supporting the utility of multiple testing and surveillance strategies to understand incidence of infection more fully, and providing information related to the possibility of short-lived immunity among nearly one-fifth of infections.

Study design

Cross-Sectional

Study population and setting

All healthcare workers employed at Azienda Sanitaria Locale public hospitals and outpatient services in Torino, Italy (n=7,457) were eligible to participate in this cross-sectional study of prevalence of IgG antibodies against the S1/S2 regions of the spike protein of SARS-CoV-2. Blood samples were collected from April 17 to May 20, 2020, and tested using capillary electrophoresis and chemiluminescence immunoassay; participants with evidence of antibodies but no previous diagnosis of COVID-19 were also asked to provide a nasopharyngeal swab to test for SARS-CoV-2 RNA.

Summary of Main Findings

Seventy-three percent (n=5,444) of eligible healthcare workers participated in the survey; their mean age was 49.4 years and 75% were women. 377 (6.9%) of participants had antibodies against SARS-CoV-2 S1/S2 spike proteins. Among seropositive participants, 176 (46.7%) reported a previous diagnosis of COVID-19. There were no differences in seroprevalence by age group and no significant differences by category of healthcare worker, although healthcare workers reporting close contact with patients had higher seroprevalence than those with limited direct contact (7.5% vs 5.2%, p=0.013). Antibody titers were significantly higher among older age groups and among those with a previous COVID-19 diagnosis. Among 201 healthcare workers with IgG antibodies but no prior COVID-19 diagnosis, 7 (3.5%) had evidence of SARS-CoV-2 RNA in their swab by RT-PCR. 17.4% of participants who reported a previous COVID-19 diagnosis had no detectable IgG antibodies.

Study Strengths

All healthcare workers in public facilities in Torino were eligible to participate, and >5,000 did so. Participants with evidence of IgG antibodies but no prior COVID-19 diagnosis were screened for viral shedding to triangulate the findings.

Limitations

Healthcare workers who declined participation could have been different than those who participated, resulting in biased estimates of overall seroprevalence among healthcare workers. The study was not powered to investigate differences between staff categories. The reported sensitivity (97.9%) and specificity (98.5%) of the assays used were high, but imperfect, like all laboratory tests.

Value added

This study reports on the seroprevalence of IgG antibodies to SARS-CoV-2 among healthcare workers in a high incidence area of Italy.

This review was posted on: 19 December 2020