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Background: The coronavirus disease 2019 (COVID-19) pandemic caused by the novel pathogen SARS-CoV-2 has spread rapidly around the world, causing massive hospitalisations and deaths, and placing an unprecedented burden on the world economy, globalisation and healthcare. Associations between characteristics of COVID-19 patients and clinical outcomes and immunity remained to be studied. This PhD work explored these associations to help understand COVID-19, inform prevention and control measures and reduce relevant burdens. Methods: This work used retrospectively collected data of all cases from 24 hospitals in Jiangsu province, China from January 10, 2020 to March 15, 2020; data from Huangshi city, Hunan province, China from January 21, 2020 to February 29, 2020; and data from a prospective longitudinal study conducted at Richmond Research Institute, St George's University of London, UK from March 19, 2020 to February 10, 2021. Adverse outcomes were severe/critical illness, disease deterioration (from asymptomatic/mild/moderate to severe/critically ill status) and respiratory failure during 14-day follow-up. Immunity status was assessed using anti-SARS-CoV-2 immunoglobulin G (IgG) levels measured repeatedly. Results: Of 625 patients in Jiangsu, 64 (10%) were severe/critically ill; 6% of patients had disease deterioration; 9% of patients had respiratory failure; and no patients died at the end of the study. Odds of being a severe/critically ill case were associated with age (year) (odds ratio [OR] 1.06, 95% CI 1.03-1.09), lymphocyte count (109/L) (OR 0.25, 95% CI 0.08-0.74), and pulmonary opacity in CT (per 5%) on admission (OR 1.31, 95% CI 1.15-1.51). Four variables were identified to be independently related to the occurrence of disease deterioration: age (year) (OR 1.08, 95% CI 1.04-1.12), pulmonary opacity score (per 5%) (OR 1.32, 95% CI 1.12-1.57), lymphocyte count (109/L) (OR 0.28, 95% CI 0.09-0.91), and imported cases (exposed to the pandemic centre) (OR 2.45, 95% CI 1.03-5.80). Age (year) (OR 1.07, 95% CI 1.03-1.10), respiratory rate (breaths/minute) (OR 1.23, 95% CI 1.08-1.40), lymphocyte count (109/L) (OR 0.18, 95% CI 0.05-0.69), and pulmonary opacity score (per 5%) (OR 1.38, 95% CI 1.19-1.61) at admission were associated with respiratory failure. Predictors including age, lymphocyte count and pulmonary opacity score were selected to develop a nomogram to predict severe COVID-19. The nomogram exhibited good discrimination (area under the receiver operating characteristic curve [AUC] 0.93, 95% CI 0.90-0.96 in the derivation cohort; AUC 0.85, 95% CI 0.76-0.93 in the validation cohort) and satisfactory agreement. Anti-SARS-CoV-2 IgG levels declined non-linearly from month 2 to 11 and may be associated with gender (female vs. male; geometric mean ratio [GMR] 4.78, 95% CI 0.99-22.98), race (Caucasian vs. other races; GMR 0.19, 95% CI 0.03-1.02) and the loss of smell and taste (GMR 9.40, 95% CI 1.12-78.97). Conclusion: Age, lymphocyte count and lung opacity scores were associated with severe COVID-19, disease exacerbation and respiratory failure, and the nomogram composed of these three factors performs well in predicting the risk of severe COVID-19. This enables physicians to identify high-risk patients early and correctly, and take corresponding proactive interventions to reduce mortality and save lives. The potential association between anti-SARS-CoV-2 IgG levels and loss of smell and taste may need to be considered when developing targeted treatment and vaccine programs to reduce severe disease. Patients reporting loss of smell and taste may have higher IgG levels and require stricter monitoring when in isolation.