C4B and COVID-19: As described for K. pneumoniae, all sera taken at the acute phase of severe COVID-19 were significantly compromised in their ability to deposit complement activation products, such as C3b, C4b and C5b-9, on the surface of S. aureus when compared to the degree of complement opsonisation seen in parallel when using NHS and convalescent sera of the same patients taken 3 months after release from hospital (Figures 6D–F).