Epidemiological and Immunological Perspectives on SARS-CoV-2

With the start of the SARS-CoV-2 pandemic in late 2019, understanding the immune responses to SARS-CoV-2 in various populations became critical for guiding public health strategies and vaccination campaigns. The variability in how e.g. different age groups, populations and individuals with prior infections as well as individuals with vaccine breakthrough infections responded to the virus and vaccines offered valuable insights in understanding the pandemic’s dynamics. This thesis synthesized data and publications from multiple cohorts to shed light on the immune dynamics of children and the general population and investigated the impact of emerging variants. Children and adolescents often experience mild symptoms during the acute phase of COVID-19, making SARS-CoV-2 infections easily overlooked in this population. The first manuscript sought to better understand the immune responses in children following infection as well as the role of children in transmission dynamics of the early SARS-CoV-2 pandemic. In a cross-sectional design, children aged 1-17 were recruited, symptoms were recorded, and antibody responses were analyzed. Of the 2,832 children tested between May and July 2020, antibodies were identified in 162 children (5.7%). Notably, only 86% of antibody-positive children exhibited sufficient neutralizing activity, irrespective of age and sex, whereas 30% of PCR positive children showed no seroconversion at all. In contrast to grown-ups, neither specific COVID-19 symptoms except for loss of smell and taste, nor any relationship between antibody response and symptoms could be found. SARS-CoV-2 antibody testing was rapidly introduced in early 2020 and played a crucial role in determining serostatus and potential correlates of protection. Yet, the transition from initial test development to exhaustive population-based testing harbors the risk of introducing spectrum bias. The second study of this thesis aimed to explore the presence and extent of spectrum bias and to estimate the performance measures of antibody tests in a population-based setting, using data from 856 participants within the TiKoCo-19 cohort. Notable differences in test sensitivities and specificities were observed when transitioning from simulated test-development to population-based application, with greater bias in earlier tests. The Roche ELECSYS N and S tests outperformed others in sensitivity and specificity for dichotomous outcomes but did not correlate well with neutralization capacity. In contrast, the in-house ELISA, though inferior in dichotomous testing, showed solid quantitative correlation with neutralization capacity, indicating it might better reflect protective immunity. In the face of hotspot outbreaks, contradicting reports on infection fatality and quickly introduced vaccination, understanding the dynamics of SARS-CoV-2 spread and the impact of public health interventions required detailed population-level data over time. Thus, in the third manuscript, a longitudinal serosurveillance study with 4203 participants in Tirschenreuth, the hardest-hit county in Germany, provided insights into seroprevalence, surveillance detection ratios (SDR), infection fatality ratios (IFR), and vaccination success. From June 2020 to April 2021, the seroprevalence increased from 9.2% to 15.4%. The SDR decreased significantly from 5 to 1, reflecting enhanced testing efforts. The general IFR of 2.3% increased slightly to 3.3% and remained highest among individuals aged 70 and above (>10%). By April 2021, vaccination rates among the elderly were high, exceeding 77.4%. The fourth manuscript aimed to broaden the knowledge on the vaccine immunogenicity. Thus, SARSCoV-2 antibody responses were measured in the Tirschenreuth cohort during April 2021. Single-dose vaccinated participants showed median neutralizing antibody titers of ID50 = 25 to 75, whereas twodose vaccinated individuals had significantly higher titers. Regression analysis indicated that older age and longer time since vaccination reduced antibody levels. Previously infected participants with one vaccine dose exhibited high median ID50 levels, suggesting robust immune priming. Prior infection also led to higher IgA levels in response to vaccination compared to uninfected individuals. The emergence of VOCs with reports of immune escape and immune imprinting as well as a cooccurring change in infection dynamics and spiking reports of vaccine breakthrough infections, further expanded the landscape of SARS-CoV-2 immune responses. The final manuscript examined the impact of SARS-CoV-2 variants of concern (VOCs) and vaccination on the neutralizing antibody profile in a longitudinal multicenter cohort study. Analysis of vaccinated and non-vaccinated individuals postinfection with Alpha, Delta, or Omicron variants showed enhanced neutralization capacity and breadth in vaccinated individuals against multiple VOCs. Sustained neutralization was observed after breakthrough infections, particularly in triple-vaccinated individuals. Antigenic mapping revealed a shift towards VOC specific responses post-infection, yet long term responses revealed an imprinted bias towards the initially immunized antigen. Nonetheless, machine-learning models could confirm a sustained change in immune profiles following breakthrough infections. In conclusion, this collection of studies shed light on the infection and antibody dynamics in children and hotspot regions during the early pandemic. We identified senior care homes and the elderly as drivers of infection fatality, underscored the variability in immune responses among different populations and highlighted the importance of vaccination. Additionally, multiple widely used tests were benchmarked, identifying apparent spectrum bias in early tests. Finally, variant based immune imprinting effects were illustrated, though bioinformatical methods could identify enhanced breadth and sustained shifts in immunity after antigenically distant immunization events