During the first pandemic wave, the dark figure of SARS-CoV-2 exposure was estimated to be high, however, an accelerated loss of antibodies was reported after about 6 months post infection. This study was performed to unveil the group of serological non-responders (NR) in PCR+ individuals 6-9 months after the first pandemic SARS-CoV-2 wave in spring 2020 and to evaluate their specific cellular immune response towards spike-molecule compared to PCR- and not PCR-tested (NT) household contact persons. SARS-CoV-2-specific antibodies were quantified using a commercial ELISA kit. The synergistic binding strength was assessed as relative avidity index (RAI) using ammonium-thiocyanate as chaotropic agent. The specific IFNγ-production in response to spike-protein was determined in spot-forming-units (SFU) by ELISPOT-assay. In PCR- 50.0%, in PCR+ 35.3% and in NT 20.7% had undetectable IgG-anti-SARS-CoV-2 and were considered non-responders (NR). All seropositive responders from the PCR-, 45.5% of PCR+ and 43.0% of NT developed high avidity (RAI>60%). In serological responders, cellular responses were detected in 75.0% PCR-, 75.8% PCR+ and 66.7% NT. In serological NR, positive SFU were found in 75.0% PCR-, 22.2% PCR+ and 17.4% NT. Significantly higher stimulation-indices were seen in PCR+ responders compared to PCR+ serological NR. Our findings showed that also PCR- and household contact persons who were not tested (NT) developed SARS-CoV-2-specific humoral and cellular immune responses. The relatively large proportion of serological non-responders but also the proportion of cellular non-responders within the group of IgG-positive individuals after PCR+ infection underlines the need for COVID-19 vaccinations in the reconvalescent group.

Level and duration of protective immunity against SARS-CoV-2 after primary infection is of crucial importance for preventive approaches. In order to provide evidence for the longevity of specific antibodies, we investigated the generation and maintenance of neutralizing antibodies of convalescent SARS-CoV-2-afflicted patients over a five month period post primary infection using an immunofluorescence assay, a commercial chemiluminescent immunoassay and an in-house enzyme-linked plaque-reduction neutralization assay. We present the successful application of an improved version of the plaque-reduction neutralization assay, which can be analyzed optometrically, significantly simplifying the interpretation of the results. Based on the results of the plaque-reduction neutralization assay, neutralizing antibodies were maintained in 85.3% of convalescent individuals without significant decay over five months. Furthermore, a positive correlation between severity of infection and neutralizing titer was shown. In conclusion, SARS-CoV-2-afflicted individuals have been proven to be able to establish and maintain neutralizing antibodies over a five months’ period after primary infection which allows to hope for long-lasting presumably protective humoral immunity after wild-type infection or even after vaccination.