To the best of our knowledge, this is the first prospective longitudinal study on SARS-CoV-2 functional seroconversion and self-perceived infection risk in frontline HCP. We show that in a country with comparably low COVID-19 prevalence and an advanced, resource-rich healthcare system, the current rate of anti-SARS-CoV-2-Ig seroconversion in HCP after the peak of the pandemic is low, while the frequency of reported respiratory symptoms and the self-perceived risk for having contracted COVID-19 is considerable.
Since the emergence of the virus in late 2019 in China, the imminent threat to HCP of contracting COVID-19 in the workplace has been a pressing issue [19]. High rates of asymptomatic infections, ranging from 18% to 88% [7, 20, 21], and transmissions before the onset of symptom [22] have raised concerns about a potentially high rate of undiagnosed SARS-CoV-2 infections. In this context, serology studies are important to characterise transmission rates, e.g. also from children without symptoms [23], and provide insight into humoral immunity to SARS-CoV-2.
Although many uncertainties regarding COVID-19 antibody testing exist, the extent of the current crisis does not allow one to wait for “guaranteed validity” of serological diagnostics [24, 25], and diagnostic algorithms have to be adjusted to different settings and regional prevalences. In this regard, our study addresses several central questions about SARS-CoV-2 transmission and serological monitoring which are relevant to regions around the globe. In areas with resource-limited healthcare infrastructure and/or infection control, the pandemic has overwhelmed HCP and healthcare systems much like the initial wave of infections in Wuhan or the Lombardy region [26, 27]. However, other countries, such as Germany, were in the fortunate position of successfully flattening the exponential spread of the virus and were able to provide hospitals and care givers with appropriate PPE in a relatively timely fashion. In spite of this, a recent national survey collecting data during the time period covered by our study reported that over 60% of German HCP, particularly women, had concerns regarding their own health while working during the current pandemic [28]. This observation is in line with our finding on self-perceived risks of having contracted SARS-CoV-2, particularly in female participants. Study participants were able to access their test results, which, as reported here, were overwhelmingly negative. This presumably contributed to the significant decline in risk perception over the study period. The high follow-up rate in the CoCo 1.0 cohort, however, supports the idea that study participants were highly interested in their personal serological status.
The observed low cumulative incidence (0.46%) for functional anti-SARS-CoV-2 IgG is based on the combination of commercially available serology assays and in-house neutralisation tests and stands in contrast to reports from Spain, Italy and UK, in which much higher seroconversion rates in HCP are reported [2, 29, 30]. In addition to the low regional prevalence of COVID-19, sufficient access and rigorous use of PPE are likely to have contributed to this outcome. The only participant with confirmed seroconversion was most likely infected in a COVID-19 hotspot outside Germany. Interestingly, evidence for differences in the rate of hospital-acquired versus community-acquired SARS-CoV-2 infections is limited even in areas with higher COVID-19 disease burden [2, 8].
Our data highlight the need for a cautious approach to serology screening and result interpretation in regions with low SARS-CoV-2 infection rates. Mass testing of both symptomatic and asymptomatic HCP has been proposed to reduce spread in mild or asymptomatic cases and to protect the healthcare workforce [31]. Nevertheless, given the differences in local spread dynamics, pre-testing probabilities and targeted screening approaches must be considered [32]. The high rate and lack of discriminative value of respiratory symptoms observed in our cohort is consistent with findings from other groups [33, 34], which show that non-respiratory symptoms in HCP (fever, anosmia/ageusia, muscle ache, ocular pain, general malaise and extreme tiredness) were associated with positive SARS-CoV-2 PCR results, while respiratory symptoms were not. Thus, non-respiratory symptoms are likely better measures of pre-test probability in symptomatic individuals.
Our results suggest that all positive results obtained by an ELISA tests from asymptomatic individuals or those with mild or unspecific symptoms should be confirmed or disproved by an alternative serology test. Combining independent serology tests to increase diagnostic accuracy for COVID-19 may be also important when assessing unusual inflammatory disease manifestations of SARS-CoV-2 infection in selected patient groups, including children and adolescents [35]. Whether this secondary, confirmatory testing should target different SARS-CoV-2 antigens or whether simply employing an alternate technique might suffice (e.g. chromatographic lateral flow rapid testing) will require further investigation. This orthogonal testing algorithm in low prevalence settings is also in line with the current recommendations for COVID-19 serology testing by the US Centers for Disease Control and Prevention (CDC) [25]. Alternatively, screening strategies may include parallel detection of immunoglobulins against other human coronaviruses (HKU1, OC43, NL63, and 229E) with high potential for cross-reactivity.
Studies in patients with COVID-19, ranging from mildly symptomatic to critically ill, have consistently shown that almost all patients have detectable antibodies by day 28 [31, 36]. All our study participants with PCR-confirmed COVID-19 with negative results for anti-SARS-CoV-2 S1 IgG were tested at least 26 days after disease onset. Interestingly, all were female with mild disease. These factors have been suggested to be associated with weaker humoral anti-SARS-CoV-2 immunity. If illness severity correlates with anti-SARS-CoV-2 IgG responses and neutralisation potency [14], we hypothesise that asymptomatic SARS-CoV-2 infection could lead to a considerable number of transient ‘viral carriers’ with undetectable systemic humoral immunity. These individuals would be missed by studies using serology screening only [11, 37]. The extent to which asymptomatic cases contribute to the current pandemic is thus far unknown, and anti-SARS-CoV-2 responses in subclinical infections, as we demonstrate here, must be carefully characterised to better assess the rate of serological non-responders. The extent to which serological data can be employed to identify previously infected pauci- or asymptomatic persons remains unknown [5]. Of note, cellular immunity against SARS-CoV-2 alone may confer protective immunity in the absence of antibody response [37, 38].
A disadvantage of functional neutralisation assays is that they can only be performed by experienced staff in a biosafety level 3 laboratory because of the need for culture live virus. The surrogate neutralisation assay we use in this study has shown close correlation when compared to assays using live pseudotyped vesicular stomatitis virus (VSV) incorporating the S protein of SARS-CoV-2 [18]. This assay consistently gave results analogous to the neutralisation assay with live SARS-CoV-2. This may become a useful tool for ascertaining robust systemic humoral immunity and assessing the kinetics of protective immunity.
Our study has several limitations. We did not perform molecular testing on respiratory specimens, which would provide information on viral carrier status in pauci- or asymptomatic HCP. We did not investigate localised immune responses, e.g. IgA in tears or mucosa fluids, or innate and cellular immune responses resulting from SARS-CoV-2 infection. Our questionnaire primarily focused on respiratory symptoms, which turned out to be of little discriminative value for identifying COVID-19. Our assessment of absolute self-perceived risk is probably a rough estimate, likely reflecting a composition of public and individual risk perception. Of note, this report represents an interim analysis, and the further CoCo cohort recruitment will likely provide more information on these topics.