Details on demography and occupation of the study participants are summarized in table 1.
At first time point (T1), a total of n1=3301 hospital employees could be enrolled whereby sociodemographic variables (see table 1) were comparable to the total AUVA population. At the second test time point (T2), which was on average 40.1±9.8 days later, a reduced number of n2=2941 could be re-tested by means of a PCR test.
Within the study cohort, 334 persons (10.1%) had comorbidities that are considered to be associated with a more severe course of disease in Covid-19 (see table 2). Among the participants with comorbidities, 257 (7.8%) had hypertension, 41 (1.2%) had diabetes mellitus, 19 (0.6%) had coronary artery disease, 43 (1.3) had COPD, and 14 (0.4%) had chronic kidney disease. These 334 individuals, together with an additional 31 (0.9%) immunocompromised persons are considered a risk group in the event of infection with SARS-CoV-2. Furthermore 1116 (33.8%) participants reported to have allergies. (table 2)
A total of 769 (23.3%) employees are smokers, whereby 513 (15.5%) smoke between six and 15 cigarettes per day, 212 (6.4%) smoke more than 15 cigarettes per day, and 44 (1.3%) stated to use other products containing nicotine. Chi-squared analysis revealed that in individuals who smoke, the incidence of flu-like symptoms within six weeks before study enrollment was significantly higher (11%) compared to non-smokers (9%, p < 0.001, Cramer V = 0.71).
Active virus carriers during the study period
Only one person had a positive PCR test result at T1 on May 12th. This participant was a medical doctor, who already tested positive 60 days before study inclusion and is therefore allocated to group A in our analysis.
Participants with positive test results
A total of nP=32 participants were tested SARS-CoV-2 positive at any time point either prior to the study (i.e. prior positive PCR-Test = Group A) or were tested positively for antibodies during the study (positive or questionable/inconclusive rapid antibody test and/or positive ELISA (Roche and/or Diasorin) = Group B). (Sociodemographic and health characteristics of these groups can be found in table 1 and table 2, respectively.)
Nineteen (59.4%) out of 32 test-positive participants reported no symptoms of an acute viral illness within six weeks prior to study enrollment as well as during the study period.
PRNT was performed if any antibody test was positive. Due to limited serum sample availability in 4 out of 32 participants, PRNT was performed on 28 sera with 25 out of which neutralizing activity was observed (one test was questionable). One participant lost neutralizing antibodies between T1 and T2 and another three participants lost them between T2 and T3.
Concerning clinical signs, rhinitis (21.9%) and loss of taste and olfactory sense were the most prominent symptoms (21.9%; table 3) in the group who tested positive at any time during or prior to study inclusion.
Group A: Participants with a positive PCR test prior to study inclusion
Fifteen employees had positive PCR test results for acute SARS-CoV-2 infection prior to study inclusion, with an average of 44.5±20.0 days between a prior positive PCR and T1 in the present study. Out of these, eight persons suffered from symptoms in the preceding six weeks.
Thirteen of these employees showed a positive neutralization test, for one participant the neutralization test result was below the threshold of a 1:4 dilution, and for another participant no blood sample could be obtained. Based on the blood samples only for seven of these participants the rapid antibody test was positive – including the person with the neutralization value below the threshold – so the sensitivity for the rapid antibody test in group A at T1 was only 46.2% (figure 3). Using the rapid antibody test with serum nine positive results (sensitivity= 69.2%) and the one participant with the neutralization value below the threshold could be detected correctly.
A total of thirteen participants had a positive ELISA- test from Roche again including the one person with the neutralization test below the threshold, resulting in a sensitivity of 92.3% when compared with the neutralization test. Based on the ELISA- test from Diasorin ten participants were positive, again including the NT negative participant (sensitivity=69.2%).
At the second timepoint (T2), a neutralization test result could only be obtained for eleven participants (all of them have tested positive at T1). The rapid antibody test based on whole blood was only positive for four of them (sensitivity = 36.0%), while based on blood serum it was positive for seven (sensitivity=64%). Concerning both ELISA tests the Roche test detected all eleven positive participants (sensitivity=100%) and the Diasorin test only nine positive participants (sensitivity=82%).
For twelve participants a neutralization test result was obtained at (T3), showing that two persons lost their antibodies between T2 and T3 (marked with T3(-) in figure 3); both ELISA tests detected all nine positive participants (sensitivity=100%), the Roche test resulted in one false positive whereas the Diasorin test detected all three negative participants correctly.
Concerning those participants where a positive neutralization test was obtained, a significant decrease of the value from 26.5±14.7 (Md=32.0) at T1 to 21.8±16.2 (Md=16.0) at T2 and to 17.8 ±9.8 (Md=16.0) at T3 could be seen (c2=10.4, df=2, p=0.006) – with an average time between T1 and T2 of 40.3±6.1 days and between T2 and T3 of 151.6 ±8.1 days.
Group B: Participants with a positive antibody test during the study, either rapid antibody and/or ELISA at T1 and/or T2
Seventeen employees were tested positive for antibodies during the study, either with the rapid antibody test and/or one of the ELISA tests, whereby the latter were only performed in individuals with a prior positive or questionable rapid antibody test. For 14 of these participants a neutralization test for validation could be performed. The majority was asymptomatic as only five participants reported symptoms in the preceding six weeks.
At T1, 14 persons of group B had a positive rapid antibody test based on whole blood, which could be confirmed for ten persons via neutralization test (sensitivity = 83.3%; figure 4). Using blood serum for the rapid antibody test, which was available only for 13 persons, all of them were positive – compared with the neutralization test but also one negative participant and one participant with a questionable result (sensitivity=100%).
The ELISA-Roche test was positive for nine persons (sensitivity=81.8%) and the ELISA-Diasorin test was positive for ten persons (sensitivity=90.9%) again compared to the neutralization test (figure 4). At T2 neutralization tests could be performed for fifteen persons; one additional person who had a negative PCR and negative rapid antibody test at T1 but reported on flu-like symptoms prior to T1 was tested positive at T2 (No 16, marked T2(+) in figure 4) and one person seemingly lost his/her antibodies in the 41 days after T1 (No 21 ,marked T2(-) in figure 4). Based on whole blood, 12 persons were tested positive with the rapid antibody test, whereby ten cases (sensitivity=83.0%) which also showed neutralization activity. Although, based on blood serum, 14 persons had positive results in the LFA, 11 of which also showed neutralization activity (sensitivity= 92.0%). The ELISA-Roche showed a sensitivity of 92% and the ELISA-Diasorin one of 100% for T2 using PRNT as reference.
Thirteen participants could be re-tested at T3 with the neutralization test showing a positive result for eleven persons. One additional participant became positive (No 17, marked with T3(+) in figure 4) which had so far only been positive in the rapid antibody test at T1 and T2, but reported flu-like symptoms between T2 and T3. Another person lost detectable antibodies (No 23, marked with T3(-) in figure 4). The ELISA-Diasorin test correctly identified all positive (sensitivity=100%) and negative participants while the ELISA-Roche test resulted in one false negative result (sensitivity=91%).
Concerning those four participants (no 17, 18, 19, 20) who are marked with an asterisk in figure 4, only the rapid antibody test was positive either at T1 and/or T2 – so it may be hypothesized that they have always been false positive in the rapid antibody test and participant no 17, who showed antibodies at T3 most probably had corona between T2 and T3.
Also, in group B a significant decrease in the neutralization test value from 37.5±29.0 (Md=32.0) at T1 to 30.1±18.6 (Md=26.7) at T2 to 24.4± 15.6 (Md=28) at T3 could be seen (c2=14.389, df=2, , p=0.001) – with an average time of 43.7±8.5 days, and 143.6±16.2 days between T1 / T2, and T2 / T3, respectively.
The rapid antibody test shows a low sensitivity of only 43.2% (T1)/36% (T2) based on blood and 69.2% (T1)/64% (T2) based on serum in a group of participants who had been PCR positive recently. But, if a positive or questionable rapid antibody test result was obtained, the result could be confirmed via PRNT at the same time of sample collection for 83.3% (T1)/83% (T2) based on blood and 100% (T1)/92% (T2) based on serum. Summarizing these findings, the ELISA-Roche shows higher sensitivity than ELISA-Diasorin in participants who have been positive a priori, but in validating the rapid antibody test results ELISA-Diasorin outperformed ELISA-Roche.
Decline of antibody concentration
PRNT values showed a significant decrease from 31.8±22.9 (Md=32.0) at T1 to 26.1±17.6 (Md=21.3) at T2 to 21.4 ±13.4 (Md=16.0) at T3 (c2=23.848, df=2, p<0.001) – with an average time of 42.4±7.7 days between T1 and T2 and 146.9±13.8 days between T2 and T3. Only two participants maintained their antibody titers at all three time points. Generally, the decrease in concentration is independent of the time period between T1/T2 and T2/T3, respectively (rT1/T2=.065, p T1/T2=0.775; rT2/T3=.042, p T1/T2=0.861).
General antibody seroprevalence
No difference in terms of antibody seroprevalence was observed between male and female participants, smokers and non-smokers, the geographical area of the participating center, or the presence of comorbidities or allergies (p>0.05). Only medical doctors were slightly overrepresented (p=0.097) in the group of positive cases, and individuals who were tested positive showed somewhat more traveling activities, especially outside Europe in February/March (p=0.010). The number of persons per household was even slightly smaller (2.34±1.5 vs. 2.73±1.32; z=1.783, p=.075) in the group of persons with antibody seroprevalence with no differences according to the number of children below the age of 15 years (p>0.05). Only seven persons reported of cohabitants with flu-like symptoms at least six weeks prior to the study whereby five had symptoms themselves (the neutralization test confirmed antibody seroprevalence for six persons and for one person the results were questionable). Also, asymptomatic persons do not differ from symptomatic ones in the neutralization test values (p>.0.05). Finally, persons with positive seroprevalence do not differ according to how they reach their work place (public transport, car, bicycle or walk; p>0.05).