The US administration recently rolled out, with great fanfare, an emergency authorization for the use of convalescent blood plasma to treat Covid-19. This is politically controversial, because it involves a short-circuiting of the traditional FDA approval process for new treatment. Aside from that though: what is the newly permitted treatment?
The liquid part of the blood, that is, the blood without the blood cells, makes up a yellow fluid known as plasma, which includes antibodies, clotting factors, and proteins.
The antibodies are the key here. People who have successfully recovered from Covid-19 have antibodies in their plasma, veterans of their successful war against the virus. The hope behind the use of the plasma in therapy is that these micro-veterans can be useful in fighting the same war on another battlefield.
Though convalescent plasma infusion has been employed against viral infections for more than a century, its efficacy in the Covid-19 context is still a matter of some controversy.
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There is a safety issue as well. One has to be extremely confident that the donor of the blood has in fact fully recovered from Covid-19 before one makes a transfusion, or the procedure could end up spreading the disease it is intended to treat.