Signals from recent several publications have suggested convalescent plasma (CP) as
an effective treatment for nonmechanically ventilated COVID-19 patients (1), despite
other trials have been inconclusive or negative (2, 3). Health systems should choose:
is it better to directly bank CP from recovered donors or instead to ship CP to industries
for transformation into pharmaceutical-grade CP or fractionation into hyperimmune
serum ? Until now, hyperimmune serum has been typically used for post-exposure prophylaxis
(e.g. tetanus, diphtheria or rabies) for infectious diseases that can be prevented
by vaccines and treated by antimicrobials, leaving no room for deployment of alternative
therapeutic CP programs. Nowadays a unique landscape is provided by advances in plasma
apheresis and plasma fractionation procedures on one side, and occurrence of a respiratory
virus pandemic for which no curative antiviral is available on the other side.
While there are now more than 80 clinical trials investigating CP therapy for COVID-19
worldwide (1) and more than 90,000 patients have been treated to date with CP in the
USA only under expanded access programs, the vast majority of health policy makers
are generally assuming this approach to be a transient stage. Figure 1
summarizes the possible routes from CP donors towards plasma-derived (combination)
therapies. Hyperimmune serum is often considered a superior, pharmaceutical-grade
product than freshly collected CP: as such, it is often believed that CP should be
replaced by hyperimmune serum as soon as it is available. In the meanwhile, several
countries are considering solvent/detergent-inactivated pharmaceutical-grade convalescent
plasma as in intermediate stage which could facilitate logistics and benefit assessment
in randomized clinical trials, especially when pharmaceutical-grade nonconvalescent
plasma is used as a control. Scalability (i.e. availability of therapeutic doses for
large number of patients) for hyperimmune serum and CP is similar, given that CP is
the source material for hyperimmune serum. Nevertheless, several additional points
show differential features, and they should be carefully analyzed before drawing conclusions.
Figure 1
Routes from convalescent plasma donation to plasma-derived medicinal products. CP:
convalescent plasma; PGP: pharmaceutical-grade plasma; PGCP: pharmaceutical-grade
convalescent plasma; PRT: pathogen reduction technologies; S/D: solvent/detergent;
HS: hyperimmune serum.
Figure 1
A first consideration is speed of access, i.e. manufacturing turn-around time. CP
collection can be implemented very early during the course of a pandemic (as soon
as a single fit donor is judged fully recovered and has relevant titers of neutralizing
antibodies), while hyperimmune serum usually requires several months to adjust fractionation
plants according to good manufacturing production (GMP) regulations SARS-CoV2 is a
recent virus. Its diversity is much lower than for other viruses, but with the rapid
massive infection of diverse human populations, major genetic variation is becoming
increasingly likely. To date, mutations within the receptor-binding domain (RBD) of
the Spike protein impacting on antibody neutralization are accumulating. If the dominant
strain changes later in the pandemics (e.g. in successive epidemic waves), the existing
hyperimmune serum bulks could be ineffective, while CP collection can be restarted
immediately and result in an effective new product. If no dominant strain emerges
but rather different strains circulate in different areas of the world at the same
time (as it is still currently the case with SARS-CoV2 (4)), hyperimmune serum manufactured
from donations collected in one continent could prove ineffective in a different continent.
Even if hyperimmune serum pools CP units from different countries, the dilution factor
is likely to make the most useful antibody specificities likely available at useless
titers within a single hyperimmune serum dose.
A second point is safety. Several countries have mandated pathogen reduction technologies
and additional molecular disease screening (MDS) on every CP donation: while these
rules make CP far more expensive, CP is nowadays at least as safe as hyperimmune serum
in terms of transfusion-transmitted infections, and exposes the recipient to a single
donor instead of thousands of donors pooled. ABO blood group incompatibility and hemolysis
due to natural isoagglutinins is also a common side effect from hyperimmune serum,
which doesn’t occur in ABO-matched CP transfusions. The risk of transfusion-related
acute lung injury in nations were previously gravid females are prevented from donations
is close to zero, and hence the risk from CP and hyperimmune serum (which also contains
anti-HLA IgG) is likely comparable (anti-HLA antibodies not currently being among
the mandatory lot release tests for hyperimmune serum). The final reinfusion volume
is hardly a great advantage, given that the 200 ml-therapeutic dose of CP is not enough
to cause circulatory overload in patients (5).
A third point is potency, i.e. efficacy at delivering clinical benefit. Hyperimmune
serum is definitively standardized to a specified neutralizing IgG content per volume,
as measured by a viral neutralization test. Nevertheless, there is currently no clearly
defined threshold for neutralizing antibody content in hyperimmune serum stocks for
COVID19. Although hyperimmune serum, due to its pooling nature, obviously contains
more antibody specificities than a single unit of CP, the dose does not necessarily
represent the best possible dose, since it reflects dilutions of a few very high-titer
donations into thousands of low-titer donations within the pool. Dilution invariably
happens in pharmaceutical-grade convalescent plasma manufacturing because of pooling.
Vendors have an obvious interest at maximizing product volume in order to increase
incomes: this could lead to inclusion of donations with very low antibody titers.
In the last pandemics it has been commonly observed that only a small fraction of
donors develops high titers of neutralizing antibodies. In other words, a single CP
unit could theoretically have a higher titer of neutralizing IgG than a standardized
hyperimmune serum dose, and this is especially relevant in COVID-19 there is a risk
for so-called antibody-dependent enhancement of infection (6). CP, but not hyperimmune
serum, contains immunoglobulins of classes other than IgG, and IgA could be especially
useful against SARS-CoV2.
Most importantly, very few studies on hyperimmune serum efficacy in the treatment
of respiratory infections have been run to date. In the setting of influenza, one
RCT of hyperimmune serum has shown efficacy only in a subcohort treated within 5 days
of symptom onset (7). Although more effective plasma fractionation technologies are
under development nowadays, the currently used, old-fashioned process causes loss
of about half the protein content (8). Most of the neutralizing antibody responses
in the IgG class have been shown to be associated with the IgG1 and IgG3 subclasses
for SARS-CoV2 (9): unfortunately, the IgG3 fraction is often depleted during industrial
fractionation (10), and its impact on PRNT titers should be carefully evaluated. Additionally,
CP can include different soluble factors expected to be beneficial such as anti-inflammatory
cytokines, or, in ABO-matched units, anti-A isoagglutinins) (expected to inhibit SARS-CoV2
entry (11)), which do not occur at all (or, in the case of isoagglutinins, occur at
lower concentration) in hyperimmune serum. Clotting factors contained in CP, but not
in hyperimmune serum, can be useful in hemorrhagic infections (such as in Ebolavirus)
or potentially detrimental in prothrombotic infections (such as COVID-19).
A fourth, relevant point is cost. Under the safest and most expensive scenario for
CP collection to date (12), the overall cost per patient of CP is likely lower than
hyperimmune serum. This assumes the cost of three 200-ml therapeutic doses of CP equals
the cost of a single nonconvalescent apheresis unit plus the cost of a pathogen reduction
kit plus the cost of additional molecular disease screening (HAV, HEV and parvovirus
B19) plus the cost of the viral neutralization test. On the other side of the coin,
the cost of hyperimmune serum also includes the cost of additional shipping to distant
locations, pooling and fractionation steps, and manufacturer’s profit. A careful analysis
should nevertheless include the benefit from additional plasma derivatives (other
than hyperimmune serum) that can likely be achieved from industrial CP fractionation,
unless CP is collected under waivers (as often happens under emergency settings).
Even a small difference in cost between a single therapeutic dose of CP vs. a single
therapeutic dose of hyperimmune serum could prove significant when the number of patients
is extremely high, such as in a pandemic. Donors from countries where remunerated
donation is forbidden could be reluctant to donate for creating private profits, and
this phenomenon could impact CP availability. Nevertheless, both hyperimmune serum
and CP will presumably cost less than the vast majority of drugs currently under clinical
trials for COVID-19 (with differences likely smaller for small chemicals and higher
for monoclonal antibodies or cell therapies).
A last point is logistics of storage, distribution and administration. Shelf-life
is very similar (2 years for CP vs. 2-3 years for hyperimmune serum), but storage
temperature under current regulations is definitively easier to achieve for hyperimmune
serum(2-8°C) than for plasma (<-25°C): nevertheless, these regulations only represent
leftovers for labile clotting factor preservation and poorly apply to preservation
of neutralizing antibodies, especially is the product has to be reinfused within a
few days and has been treated with a PRT. Delivery route also favors hyperimmune serum:
while CP can be administered only intravenously, hyperimmune serum can also be delivered
intramuscularly.
Searching the databases of published research, we were not able to find any pharmacoeconomics
or efficacy study comparing hyperimmune serum vs. CP for any pathogen. We feel that
rigorous analysis accounting for the 5 abovementioned points, and eventually a randomized
trial comparing hyperimmune serum with CP, should be run before health authorities
endorse industry support and decide which fraction of CP should be addressed to plasma
fractionators.
Conflict of interest
We declare we don’t have any conflict of interest related to this manuscript.
Funding
We declare no external funding was received.
Authors' contributions
D.F. and M.T revised the literature and designed the viewpoint structure. D.F. wrote
the first draft. F.M. and G.A. critically revised the manuscript. All the authors
approved the final version.