Background: SARS-CoV-2 is a novel coronavirus, not encountered before by humans. The wide spectrum of clinical expression of SARS-CoV-2 illness suggests that individual immune responses to SARS-CoV-2 play a crucial role in determining the clinical course after first infection. Immunological studies have focussed on patients with moderate to severe disease, demonstrating excessive inflammation in tissues and organ damage. We have studied the individual response to SARS-CoV-2 of asympromatic, mild and severe COVID-19 patients in order to investigate the role of innnate and adaptive immunity in determining the clinical course after first infection. Methods: To understand the basis of the protective immune response in COVID-19, we performed a longitudinal follow-up analysis of innate and adaptive immunity in 64 adults with a spectrum of clinical presentations: (28 healthy SARS-CoV-2-negative contacts of COVID-19 cases; 20 asymptomatic SARS-CoV-2-infected cases; 8 patients with mild COVID-19 disease and 8 cases of severe COVID-19 disease). Results: Our data show that high frequency of NK cells and early and transient increase of specific IgA and, to a lower extent, IgG are associated to asymptomatic SARS-CoV-2 infection. By contrast, monocyte expansion and high and persistent levels of IgA and IgG, produced relatively late in the course of the infection, characterize severe disease. Modest increase of monocytes and rapidly declining antibodies are detected in mild COVID-19. Conclusions: The importance of innate NK cells and the short-lived antibody response of asymptomatic individuals and patients with mild disease suggest that only severe COVID-19 may result in protective memory established by the adaptive immune response.