Perceived versus proven SARS-CoV-2-specific immune responses in health-care professionals

There have been concerns about high rates of thus far undiagnosed SARS-CoV-2 infections in the health-care system. The COVID-19 Contact (CoCo) Study follows 217 frontline health-care professionals at a university hospital with weekly SARS-CoV-2-specific serology (IgA/IgG). Study participants estimated their personal likelihood of having had a SARS-CoV-2 infection with a mean of 21% [median 15%, interquartile range (IQR) 5–30%]. In contrast, anti-SARS-CoV-2 IgG prevalence was about 1–2% at baseline. Regular anti-SARS-CoV-2 IgG testing of health-care professionals may aid in directing resources for protective measures and care of COVID-19 patients in the long run.

in frontline health-care professionals (HCP) in combination with a questionnaire about respiratory symptoms and risk perception. As testing system, we employed a semiquantitative ELISA [EUROIMMUN Medizinische Labordiagnostik, Lübeck, Germany-CE certified version: specificity 99.0%, sensitivity 93.8% after day 20 according to the manufacturer [5]]. We confirmed the specificity in a set of 156 sera from non-European refugees and migrants [7] collected in 2015 as negative controls (mean age 31.6 years, range 18-67 years, 78% male). All but one tested negative for SARS-CoV-2 IgG (specificity 99.3%) and 2 out of 90 tested equivocal positive for IgA (specificity 97.8%). The serum of 18 patients after recovery of PCR-confirmed COVID-19 served as positive controls or to generate a standard curve (mean age 44.8 years; mean duration of symptoms 11.8 days, range 3-35 days; mean time since start of symptoms 30.4 days, range 21-61 days). 16/18 tested positive (n = 1 equivocal positive) for SARS-CoV-2 IgG (sensitivity 90%) and 15/18 positive for SARS-CoV-2 IgA (sensitivity 85.7%). Interestingly, the duration of symptoms as a surrogate for disease severity correlated significantly with the IgG ratio (extinction of sample to calibrator ratio) of the SARS-CoV-2 IgG ELISA (Fig. 1a).
Between March 23 and April 17, n = 217 HCP from emergency rooms, infectious and pulmonary disease wards, ICUs, pediatric departments and other units involved in COVID-19 patient care at our university hospital were included in the study. The mean age of participants was 36.5 years (range 18-63 years), and 65% were female. Most of them worked as physicians (53.5%), nurses (27.6%), or medical assistants (9.2%). The majority of participants had direct contact with patients with infectious respiratory diseases working in the emergency department (40.1%), general ward (31.8%), or outpatient departments (13.8%). At baseline, 1.6% of included personnel reported to have visited regions with high SARS-CoV-2 prevalence as defined by the German National Institute of Public Health (Robert Koch Institute [8]), 16.1% reported to have had contact with confirmed COVID-19 cases, and more than one-third (39.2%) to have had contact with suspected COVID-19 cases. 45.2% of HCP reported to suffer from at least one respiratory symptom of any severity, and 29.0% reported to have had a respiratory infection during the past 2 weeks.
Upon enrollment, study participants were asked to estimate their personal likelihood of having had a SARS-CoV-2 infection (How high do you rate the probability of having been infected so far? 0-100%). Only 12% of the n = 201 study participants, who answered this question, rated a 0% chance of having already had contracted SARS-CoV-2, while 19% rated their probability greater than 50%. Strikingly, the mean percentage of self-perceived positive SARS-CoV-2 infection status was 21% (range 0-90%, median 15%, IQR 5-30%). Male participants rated their infection risk lower than female participants (mean 16.2% vs. 23.7%, F = 4.4, p = 0.02, median 10% vs. 20%, Z = 3.4, p = 0.001) and older subjects reported lower infection probabilities as compared to younger participants (Pearson − 0.33, p = 0.004) (Fig. 1b). Reported contact to confirmed or suspected COVID-19 cases did not have a significant impact on perceived probability of infection.
In contrast to the high percentage of self-perceived positive SARS-CoV-2 infection status, only two of n = 217 tested frontline HCP showed a clearly positive reaction in the ELISA, and two displayed equivocal positive results according to the manufacturer's interpretation. Both positive results were about 20-fold lower as compared to one patient with severe COVID-19 (Fig. 1a). The majority of participants (n = 214) had no evidence of anti-SARS-CoV-2 IgG. Anti-SARS-CoV-2 IgA was positive/equivocal positive in n = 9 and n = 10 subjects, respectively, and combined IgG and IgA-positive/equivocal positive in three subjects. Altogether, anti-SARS-CoV-2 IgG prevalence was in the range of 1-2% and the self-perceived likelihood of SARS-CoV-2 infection in these individuals similar to the entire cohort. Our data on SARS-CoV-2 IgG is only partially representative for our university hospital, not fully representative for other clinics, and we do not know the source of infection in anti-SARS-CoV-2 IgG-positive HCP. However, the gap between perceived risk and evidence for an infection is most likely a phenomenon in many health-care settings. Additionally, we have only limited information about the full validity of anti-SARS-CoV-2 serology tests for screening. In a setting with low COVID-19 prevalence, the use of the spike protein S1 to screen for anti-SARS-CoV-2 IgG may be suboptimal, and testing for antibodies against, e.g., the receptor-binding domain of SARS-CoV-2 could increase sensitivity. Interestingly, given the significant association between disease severity and anti-SARS-CoV-2 IgG in our ELISA, we hypothesize that asymptomatic seroconversions could lead to numerous equivocal positive ELISA results (Fig. 1a), which still may represent neutralizing activity [9]. Such data may be difficult to interpret in cross-sectional studies and our longitudinal study design combined with neutralization assays will be informative about the magnitude of ELISA result changes over time. Finally, a matter of debate remains whether serological tests can also inform about COVID-19-specific immunity. Preliminary studies in rhesus macaques suggest that reinfection does not occur after survival of COVID-19, supporting the notion of at least temporary immunity after primary infection [10].
Taking these limitations into account, our data point toward a currently low rate of SARS-CoV-2-specific IgG in HCP in Northern Germany hospitals, where no overflow of COVID-19 patients has challenged the health-care system so far and confirmed outbreaks are limited. This is in stark contrast to the relatively high rate of self-estimated SARS-CoV-2 infection probability of our hospital's frontline HCP and strikingly different from currently high infection rates in medical personnel from Italian regions [1]. Also, our data show that personal risk perception correlates to age and sex, which should be taken into account when advising hospital staff on protective measures against COVID-19. Regular anti-SARS-CoV-2 IgG testing of health-care workers may aid in monitoring the pandemic, assessing the quality of immune responses, and directing resources to assure COVID-19 care in the long run.