Combined Prospective Seroconversion and PCR Data of Selected Cohorts Indicate a High Rate of Subclinical SARS-CoV-2 Infections-an Open Observational Study in Lower Saxony, Germany

Abstract

Despite lockdown measures, intense symptom-based PCR, and antigen testing, the SARS-CoV-2 pandemic spread further. In this open observational study conducted in Lower Saxony, Germany, voluntary SARS-CoV-2 PCR tests were performed from April 2020 until June 2021, supported by serum antibody testing to prove whether PCR testing in subjects with none or few symptoms of COVID-19 is a suitable tool to manage the pandemic. In different mobile stations, 4,817 subjects from three different working fields participated in the PCR testing. Serum antibody screening using the SARS-CoV-2 ViraChip IgG (Viramed, Germany) and the Elecsys Anti-SARS-CoV-2 assay (Roche, Germany) was performed alongside virus neutralization testing. Subjects were questioned regarding comorbidities and COVID-19 symptoms. Fifty-one subjects with acute SARS-CoV-2 infection were detected of which 31 subjects did not show any symptoms possibly characteristic for COVID-19. An additional 37 subjects reported a previous SARS-CoV-2 infection (total prevalence 1.82%). Seroconversion was discovered in 58 subjects with known SARS-CoV-2 infection and in 58 subjects that never had a positive PCR test. The latter had a significantly lower Charlson Comorbidity Index, and one third of them were asymptomatic. In 50% of all seroconverted subjects, neutralizing serum antibodies (NAbs) were detectable in parallel to N/S1 (n = 16) or N/S1/S2 antigen specific antibodies (n = 40) against SARS-CoV-2. NAb titers decreased within 100 days after PCR-confirmed SARS-CoV-2 acute infection by at least 2.5-fold. A relatively high rate of subclinical SARS-CoV-2 infections may contribute to the spread of SARS-CoV-2, suggesting that in addition to other intervention strategies, systematic screening of asymptomatic persons by PCR testing may significantly enable better pandemic control. IMPORTANCE Within this open observational study, repeated PCR (n > 4,700) and antibody screening (n > 1,600) tests were offered in three different working fields. The study identified 51 subjects with acute SARS-CoV-2 infection and 37 subjects reported to have had a positive PCR test taken externally. Thirty-one of the 51 subjects did not display any symptoms prior to testing. In addition, 58 subjects without PCR-confirmed SARS-CoV-2 infection were identified by seroconversion. Subjects, that had undergone SARS-CoV-2 infection without having noticed, more often had a low grade of immunization with no NAbs, but may have relevantly contributed to the spread of the pandemic. Based on these results, we suggest that both regular PCR and rapid test screening of symptomatic and asymptomatic individuals, specifically within groups or workplaces identifiable as having close quarter contact, thus increased infection transference risk, is necessary to better assess and therefore reduce the spread of a pandemic virus.

Keywords: COVID-19; PCR; SARS-CoV-2 antibody screening; antibody screening tests; neutralizing antibodies; subclinical cases; working groups.

FIG 1

Schematic displaying all subjects in the study undergoing PCR tests with positive and negative results and all subjects with antibody tests with positive or negative results. Twenty-six subjects with a positive PCR test did not participate in antibody screening (“dropout”).

FIG 2

Schematic depicting age and gender distribution among all participating groups of the study. Three groups were defined: (i) education/culture with employees out of universities, schools, city administration of Hannover, theater; (ii) company with employees from a biotechnological company in Goettingen; and (iii) nursing homes with residents and employees of two nursing homes in Hannover.

FIG 3

Correlation between clinical status of subjects at the time point of positive PCR test and the Ct-value as corresponding factor of the virus load. All subjects had been asked whether they had experienced any of the following possibly characteristic COVID-19 related symptoms: cough/snuff, fever/shivering, sore throat, headache, limb pain, fatigue, shortness of breath, diarrhea, and smell/taste loss (Table S1).

FIG 4

Numbers of seroconverted male and female subjects with PCR-confirmed SARS-CoV-2 infection (orange, female = white, male = striped), recognized by PCR test, in each group (education/culture; company; nursing homes) and numbers of seroconverted male and female subjects despite no previous positive SARS-CoV-2 PCR test (female = white, male = striped). Not seroconverted female subjects with PCR-confirmed SARS-CoV-2 infection are displayed as dotted column.

FIG 5

Time related occurrence of antigen-specific antibodies (N, S1, S2) as well as neutralizing antibodies (NAbs) after first positive PCR test for SARS-CoV-2 in days.

FIG 6

Charlson Comorbidity Index for subjects within the groups “educational/culture,” “company,” and “nursing homes” as well as the subgroups of subjects with and without PCR-confirmed SARS-CoV-2 infection who have seroconverted. Statistical significance was calculated using Mann-Whitney-U test.

FIG 7

Virus neutralization test results of each serum sample based on antigen-specific antibody patterns obtained by Elecsys and ViraChip.

FIG 8

Detection of antigen specific antibodies (N, S1, S2) in all PCR-positive subjects (orange) and in all subjects without a recorded positive SARS-CoV-2 PCR test (green). A positive virus neutralization-test result is depicted by a hatched overlay.

FIG 9

Virus neutralization titers (VNT) of neutralizing antibodies over time were expressed as the reciprocal of the dilution that gave a 50% reduction of stained cells. Samples from subjects with more than one sample are highlighted and connected with dotted lines.

FIG 10

Percentage of subjects with either none, 1, 2 to 4 or 5 to 8 COVID-19 typical symptoms among all seroconverted subjects without PCR-confirmed SARS-CoV-2 infection. All subjects had been asked whether they had experienced any period since the onset of the corona crisis with at least one of the following symptoms, which the subjects themselves related to a possible SARS-CoV-2 infection: cough/snuff, fever/shivering, sore throat, headache, limb pain, fatigue, shortness of breath, diarrhea, and smell/taste loss (Table S1).