For Donors:

Microtiter plates will be coated overnight at 4°C with 4 μg/mL recombinant SARS-CoV-2 RBD
(receptor binding domain) proteins (50 μL per well). The plates will be washed 3 times with
phosphate-buffered saline (PBS) containing 0.1% vol/vol Tween-20(PBST) and blocked with
blocking solution (PBS containing 2% wt/vol nonfat dry milk) for 2 hours at 37 °C. The plates
will be then washed with PBST. The serum samples will be diluted to 200-fold into PBS as
initial concentration, and serial 3-fold dilutions of serum will be added to the wells and
incubated at 37 °C for 60 minutes. After 3 washes, 100 μL of horseradish
peroxidase-conjugated goat anti-human IgG (for IgG antibody titer detection)and IgM (for IgM
antibody titer detection) antibodies solution will be added to each plate, respectively, and
incubated at 37 °C for 60 minutes. After 5 washes, 100 μL of tetramethylbenzidine substrate
will be added at room temperature in the dark. After 15 minutes, the reaction will be stopped
with a2MH2SO4 solution (sulfuric acid). The absorbance will be measured at 450nm. All samples
will be run in triplicate. The IgG titers will be determined by endpoint dilution.

Serum Neutralization Assay Vero cells (104) will be seeded 24 hours before the infection in a
96-well plate. On the day of infection, the cells will be washed twice. Serum samples from
patients will be incubated at 56 °C for 30 minutes and then diluted 2-fold in cell culture
medium (modified eagle medium). Aliquots (40 μL) of diluted serum samples (from2-fold to
2056-fold) will be added to 50 μL of cell culture medium containing 50 times the tissue
culture infective dose (TCID50) of the virus strain on a 96-well plate and incubated at 37 °C
for 2 hours in CO2 5% vol/vol. Virus antibody mix will be added to cells in 96-well plates
and plates will be incubated at 37 °C with a microscopic examination for cytopathic effect
after 5-day incubation. The highest dilution of serum that showed inhibition activity of
SARS-CoV-2 will be recorded as the neutralizing antibody titer. Assays will be performed in
triplicate with negative control samples from healthy volunteers.

For recipients:

The serum of each recipient will be obtained and enzyme-linked immunosorbent assay (ELISA)
and neutralizing antibody titers will be tested one day prior to the convalescent plasma
transfusion. Changes of Receptor Binding Domain-Specific IgG titre and neutralizing antibody
titers before and after convalescent plasma transfusion in patients will be obtained on day
0, day 1, day 3 and day 7. if possible.

All included patients would be randomized to receive either standard medical therapy
(supportive therapy) with random donor plasma versus convalescent plasma and standard medical
therapy Clinical information of all enrolled patients including symptoms at presentation,
time to presentation to the hospital and development of pulmonary symptoms would be recorded.
The details of comorbid diseases as measured by the Charlson index of comorbidity and Acute
Physiology and Chronic Health Evaluation II (APACHE II). Details of cross-sectional imaging,
chest-x-ray, bacterial or fungal co-infections and details of antibiotic treatment would be
recorded. Development of complications including acute kidney injury, acute coronary
syndrome, myocarditis, acute respiratory distress syndrome, and nosocomial infection will be
recorded. The use of high-flow oxygen, non-invasive and invasive ventilation will follow
standard guidelines and will be recorded. The details of antiviral treatment including oral
oseltamivir, hydroxychloroquine, and use of intravenous steroids will be recorded for all
enrolled patients.