New insights into protective immunity against endemic human coronaviruses

Human Coronavirus (HCoV) NL63, HCoV-229E, HCoV-OC43, and HCoV-HKU1 are coronaviruses that cause seasonal mild respiratory infection in humans. Collectively, they are known as endemic (or seasonal) HCoVs. These four HCoVs have been around in human population for centuries, and therefore they could serve as a model of the endemicity of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). This thesis explored the frequency and disease manifestation of endemic HCoV (re)infections in immunocompetent adults and people living with HIV-1 (PLWH), as well as the host factors that potentially influence such reinfections. Interval length between endemic HCoV reinfections did not appear to influence disease manifestation at the subsequent infections in adults, and the incidence of reinfection did not differ significantly between them and PLWH. Previous exposure to HCoV-OC43 appeared to reduce the risk of subsequent infection by SARS-CoV-2 at the start of Coronavirus Disease 2019 (COVID-19) pandemic. Furthermore, in a population with acute lower respiratory tract infection (LRTI), the majority of HCoV-HKU1-infected people lacked a typical serum immunoglobulin G (IgG) rise compared to people infected by other HCoVs, and their illness was rated as mild as well. Lastly, in the same population of people with acute LRTI without significant serum IgG rise, their nasal immunoglobulin A (IgA) were significantly higher at the first week of the illness compared to people with a typical serum IgG rise in response to the infection.