Background: Due to the substantial proportion of asymptomatic and mild courses many SARS-CoV-2 infections remain unreported. Therefore, assessment of seroprevalence may detect the real burden of disease. We aimed at determining and characterizing the rate of SARS-CoV-2 infections and the resulting immunity in a defined population. Methods: CoNAN is a population-based cohort study in the previously quarantined community Neustadt-am-Rennsteig, Germany six weeks after a SARS-CoV-2 outbreak with 49 cases identified by PCR screening of all 883 inhabitants. The primary objective of the study was to assess SARS-CoV-2 antibody seroconversion rate using six different IgG detecting immunoassays. Secondary objectives of the study were: i .) to determine the rate of seroconversion in children; ii .) to determine potential risk factors for symptomatic vs. asymptomatic Covid19 courses; iii .) to investigate the rate of virus persistence. Findings: We enrolled 626 participants (71% of the community population). All actual SARS-CoV-2 PCR tests were negative; while a total of 8·4% (52 of 620 tested) had antibodies against SARS-CoV-2 in at least two independent tests. Twenty of the antibody positive participants had previously a positive SARS-CoV-2 PCR. On the contrary, of those 38 participants with SARS-CoV-2 infection, only 20 (52·6%) were antibody positive. Interpretation: Several antibody tests conducted six weeks after an outbreak of SARS-CoV-2 did not detect all previously PCR-positive tested individuals. Cautious evaluation of antibody testing strategies to assess immunity against the infection is warranted. Funding: CoNAN was funded by the Thuringian Ministry for Economic Affairs, Science and Digital Society (TMWWDG).

Rapid vaccination may be of benefit in long-term care facilities (LTCF) that are affected by an ongoing COVID-19 outbreak. However, there are concerns regarding the safety and effectiveness of such an approach, particularly regarding the vaccination of pre-symptomatic patients. Here, we report the effectiveness of vaccination in a German LTCF hit by an outbreak that was detected 5 days after the first vaccine doses were administered. In detail, 66.7% of the unvaccinated patients experienced an unfavorable course; this proportion was much lower (33.3%) among the vaccinated patients. Even though this study is limited by a small number of patients, the observation and the comparison with related published data shows that vaccination (i) is safe and (ii) may still be of benefit when given shortly before an infection or even in pre-symptomatic LTCF-patients.

We present here a 64-year-old male participant of the CoNAN study who experienced a PCR-confirmed mild SARS-CoV-2 infection but did not develop any measurable antibody response. Additionally, after vaccination with ChAdOx1 (AstraZeneca, Cambridge, UK) 11 months later, no antibodies were detected in six serological tests three weeks after the vaccination. When we assessed T-helper (Th) cell immunity, SARS-CoV-2-specific Th cells produced detectable amounts of IFNγ and TNF six weeks after the infection. A robust T-cell immunity remained detectable at least until six months after the infection and was boosted by the vaccination thereafter. This case report points out that an assessment of a prior infection or a vaccine response based solely on antibody detection might have limitations in individual patients.