PROS1 and COVID-19: Moreover, two observations noted above raise questions about the causal relationship between antibodies and severity of infection: NAb responses were faster in the patients who later died than in those who recovered (Zhang et al., 2006; Ho et al., 2005; Liu et al., 2019), and older patients who had recovered from mild COVID-19, had significantly stronger NAb and S-protein-binding antibody responses than younger ones, whereas higher age is a major risk factor for lethal COVID-19 (Wu et al., 2020).