Up to now (July 2020) around 10 million people worldwide have been identified as infected with SARS-CoV-2, of which nearly 500.000 succumbed to COVID-19 (1) . In addition to China, early outbreaks took place in South Korea, Japan, and Central Europe where by now, the spread of the virus could be contained with great efforts. However, COVID-19 had been declared a pandemic by the WHO on March 10, 2020 (World Health Organization, 2020) and many countries like the USA, Russia, Brazil, and India are still showing rising daily case numbers.
Health care workers are at exceptionally high risk of infection as they work on the frontline of this pandemic (2) . Perinatal centers seem to be an underestimated hotspot due to the increased presence of young, often paucisymptomatic patients, high aerosol exposure in the delivery room, and a multidisciplinary care team that requires the close proximity of staff members from multiple hospital departments such as anesthesiologists, midwifes, obstetricians, nurses and others (3).
In a well described outbreak at our University perinatal center in Regensburg, Germany, a total of 36 staff members were confirmed virus RNA-positive by reverse transcription, followed by real-time (RT)-PCR and 34 developed COVID-19 (4) . Our previously reported J o u r n a l P r e -p r o o f observations and initial antibody testing showed that 2-3 weeks after the initial PCR-based screening only a limited number of staff members affected by COVID-19 had developed relevant antibody responses (48.4%). At this time point, very few staff members who were in contact with diseased co-workers but tested negative in the PCR-test, showed any antibody response which was limited to IgA (8.2%) and in one case, to borderline elevated IgG (6) .
The objective of this follow up study was now to investigate the progression of the immune response approximately 12 weeks after the outbreak in our hospital staff of COVID-19 affected and contact persons.
The study was designed as a prospective cross-sectional study with optional longitudinal analysis, focused on immune response to SARS-CoV-2 in health care workers approximately 12 weeks after a COVID-19 outbreak in a large University children's and maternity hospital.
Details of the outbreak have been reported elsewhere (4) . Briefly, extensive RT-PCR testing was performed on hospital employees (n=379) initially and after the first serological test was commercially available, all employees were offered a voluntary participation in a SARS-CoV-2 serological test.
All study subjects were categorized into four groups according to their state of infection or exposure to SARS-CoV-2 positive individuals as previously reported (6) weeks) have already been reported (6) . All participants in the first assessment were invited to participate in the second assessment. As all data in both studies were fully anonymized and only accessible at an individual level to the participant, data from the first study could only be connected to the second study with the individual approval of the study participants and under the condition that the initial personal study log-in was still remembered by the participant. Therefore, longitudinal analyses were possible in 127 of the 166 participants of the second assessment. The study was approved by the Ethics Committee of the University of Regensburg (file-number: 20-1767-101).
Specific antibody response to SARS-CoV-2 was now evaluated by the use of two different commercially available test kits -the Anti-SARS-CoV-2 IgG and IgA ELISA (EUROIMMUN AG, Lübeck, Germany; https://www.euroimmun.com) and the Elecsys Anti-SARS-CoV-2 (Roche Diagnostics, Rotkreuz, Switzerland; https://diagnostics.roche.com).
Both tests were performed according to the manufacturer's protocol. For EUROIMMUN kits, reagent wells were pre-coated with recombinant structural protein (S1 domain) of SARS-CoV-2 for the IgA and IgG assay as previously reported (6) . According to manufacturer's recommendations, for the IgA and IgG assay an OD ratio of > 1.0 was considered positive.
The qualitative Elecsys Anti-SARS-CoV-2 Antibody immunoassay analyzer was used. The assay does not discriminate between the antibody type(s) present and can detect IgA, IgM, and IgG. The test is based on a recombinant nucleocapsid (N) antigen and has a threshold value of 1.0. Accordingly, all samples with a value < 1.0 were considered negative.
Characteristics of the study cohort are presented using descriptive statistics. Fisher's exact test was used to examine group differences. For the comparison of the proportions of participants who showed antibody responses over time the McNemar test for paired samples was used. To analyze correlation between the two different antibody tests and for correlation of antibody test results with symptoms, Spearman's rank correlation coefficient or coefficient of determination was used, where applicable. All analyses were performed using SPSS 26 (IBM SPSS Statistics for Windows, USA)
Regensburg, Germany, serologic testing was performed 2 to 4 weeks after the outbreak as previously described (6) Table 1 . (8) . Multisystem inflammatory disease is a rare complication of COVID-19 that typically affects children, but has recently been reported to also affect young adults (8, 9) .
Results of the Euroimmun IgG ELISA and the Roche immunoassay correlated well, while this was not the case for comparison of the IgA ELISA with the Elecsys immunoassay. These results are in line with other studies confirming good sensitivity and specificity of the IgG ELISA and the Elecsys Immunoassay (13, 14) . Our longitudinal testing with the same assay (Euroimmun) allowed comparison of the two time points. The additional test with the Elecsys assay confirmed the results of seroconversion and further added to the sensitivity of early detection of humoral responses.
As expected, the sequential study of IgA antibody levels showed that in the COVID-19 group, the rate of patients with detectable IgA antibodies decreased significantly over time.
This finding supports the hypothesis already described by other authors, that IgA is not an adequate marker for the long-term course of an immunological response after COVID-19 infection and may only play a role in patients with mild symptoms, especially since our COVID-19 cohort was only affected by mild to moderate symptoms (10) .
Further, also the number of experienced symptoms of the COVID-19 patient group did not correlate with immune response. None of our study participants experienced severe symptoms or symptoms correlated with a multisystem inflammatory disease. In this respect, the lack of correlation of the number of symptoms with IgG titers might be explained, but the low number of COVID-19 positive patients in our study population must also be taken into account.
We had initially speculated that exposure to colleagues with COVID-19 might lead to an antibody responses in some hospital staff dependent on the degree of exposure. Interestingly, our results showed no antibody responses in the contact group, rejecting the hypothesis of silent, antibody dependent immunization in this cohort. These results regarding the immune response of COVID-19 contact subjects are in line with the first reports on development of antibody response to COVID-19 (11, 12) . In addition, the significant decrease in IgA levels over time in the contact group fits with reports of unspecific IgA reaction to common cold coronaviruses (10, 13) .
The fact that we did not identify additional seropositive cases in our study cohort 8-12 weeks after the outbreak also allows the conclusion that no new infections have occurred during the study period, although COVID-19 patients have been treated in our hospital throughout the study period. More importantly, this result underlines the effectiveness of the applied hygiene measures to slow the spread of the virus.
For our clinical work we can draw some encouraging results from this study. Firstly, we see that the immunological response of COVID-19 patients has increased significantly over time, and we can therefore assume an increase in immunity. Secondly, we found that none of our hospital staff developed COVID-19 after increased hygiene measures have been applied as described previously (5) . This finding confirms the conclusion, that maintaining service in an obstetric unit during an outbreak is feasible when personal protective equipment, workspace distancing and good hand hygiene are implemented (3, 15, 16) . Further, the details of exposure to COVID-19 have been assessed at the time of the outbreak and are less reliable over time, due to possible exposure outside the hospital.
We conclude from our data that immune response after a COVID-19 outbreak increases significantly over time but still approximately 22% of COVID-19 patients did not mount a measurable serologic immune response within 60 days after symptoms have occurred.
Additionally, we can summarize that exposed co-workers did not develop any relevant IgG antibody levels over time. Maintaining clinical services using protective measures seems to be safe for employees and patients, even in an outbreak situation, if the necessary measures are taken fast and ferociously. Use of these protective measures need to be continued as neither immunity after infection nor herd immunity are reliable.
Flow of participants throughout the study with time periods of serologic testing and applied tests including the respective numbers. Correlation of initially reported symptoms and anti-SARS-CoV-2-IgG ratios for COVID-19
patients. The number of ten of the most common symptoms seen with COVID-19 ("COVID-19 symptoms [n]", runny nose, sore throat, headache, exhaustion/fatigue, muscle aches, anosmia, shortness of breath, coughing, fever, diarrhea and other (10) is correlated with its respective anti-SARS-CoV-2-IgG ratio ("IgG ratio") at the time points "baseline" (left) and "follow up" (right) for each participating health care worker ("HCW") initially suffering from COVID-19 with longitudinal follow up (n=25). The p-value and the Spearman's rho are depicted above each chart.
J o u r n a l P r e -p r o o f 