This episode was filmed on May 19, 2020.

If we have more recent videos about the pandemic, you can find them in our COVID-19 playlist, which we will link in the top of the description. [♪ INTRO]. None of us like to see the numbers of cases and deaths from COVID-19 continue to climb.

But alongside those grim statistics is another, slightly better one: those who have been infected and recovered. And thanks to a technique that's more than a century old, those recovered patients may be in a position to help the rest of us -- with to their blood plasma. We've successfully used transfusions of blood serum containing antibodies to a given disease as a treatment since the late 1800s.

And it could work now, because the plasma of patients who have recovered from COVID-19 -- or any viral illness, really -- should be chock full of antibodies that recognize that virus. Our immune system produces these antibodies in order to recognize and bind to viral antigens, or proteins on the surface of a virus. Even after an infection is over with, some of these antibodies remain behind in our blood.

In the 1890s, researchers in Berlin discovered that they could transfer these remaining antibodies from experimental animals to patients via blood plasma -- the fraction of our blood that doesn't include red and white blood cells. They first tested this on animals by transferring plasma from guinea pigs that had recovered from diphtheria to guinea pigs who were sick with it. The piggies recovered once they received the donor plasma.

Plasma transfusions work by conveying passive immunity to the recipient of the transfusion. Because the donor's body has created an army of super specific antibodies, they can go straight to work in the recipient -- without waiting for their body to make their own. Researchers quickly moved on from guinea pigs to adapt this new treatment for humans.

Horses were used as the plasma donors, because of their relatively mild reaction to diphtheria -- and their high volume of blood. Blood was extracted from a horse that had mounted an immune reaction to diphtheria. Then, over the course of several days, the plasma was purified and injected into the recipient.

This revolutionary procedure was so effective that it slashed the death rate from diphtheria everywhere it was used -- it's estimated to have saved 45,000 lives a year in Germany alone. Emboldened by their success with diphtheria, physicians in the early 1900's expanded the treatment to use against other diseases -- such as measles and the 1918 Spanish flu pandemic -- using plasma from either animals or recovered human patients. But while this approach was effective, it was far from perfect.

It was common for patients to develop an allergic reaction to the plasma itself. Also, since the antibodies aren't made by the recipient's body, what you get is what you get. They don't stick around after the patient has recovered, so it's only a temporary solution.

What's more, researchers in the 1940's discovered that blood itself can carry infections, and those infections could be transmitted between humans through transfusions. All that meant the sun was setting on plasma transfusions, in favor of something safer and longer-lasting: vaccines. Vaccines convey active immunity -- they work by training our own immune systems to produce antibodies against a given invader.

That primes our immune system to go on the offensive if it ever encounters the actual disease. At least, for a period of time -- some vaccines do need to be re-upped with boosters. But we don't yet have a vaccine against SARS-CoV-2, the virus that causes COVID-19.

So blood plasma transfusions may be the ticket to bridging the gap. Technology has come a long way, and we can now screen blood to make sure it's not carrying infectious diseases. Plasma transfusions have already been shown to be effective against other coronavirus diseases, such as SARS and MERS, and early reports seem to show that's true for COVID-19 as well.

Hospitals have already begun using convalescent plasma as a compassionate use treatment -- a last resort treatment for critically ill patients. And results from early clinical studies are promising. One study of ten critically ill adults, published in April of 2020, showed that patients' conditions improved, on average, within a week.

The researchers found high levels of antibodies in the patients' serum, and three of the ten patients were discharged from the hospital by the conclusion of the study. And for doctors looking to use this treatment, the technology already exists -- it's just a matter of access to plasma. Plasma can be drawn from recovered COVID-19 patients using the same plasma separation technology already in place at blood banks.

Potential donors need to adhere to their country's guidelines when donating plasma. In the US, for example, they have to have been tested positive for COVID-19 and be symptom-free for 14 days. There are some issues that still need to be addressed -- for example, early unpublished studies suggest some people might make more antibodies than others, and the blood plasma has to be the right match, because even plasma has to be compatible with the recipient's blood type.

Though everyone in that study of ten people got better, other studies show positive outcomes are not guaranteed, so more data is needed to say for sure that this is an effective treatment. Fortunately, as we receive more convalescent plasma donations, more opportunities become available to test its effectiveness. Blood centers around the US are already seeing increased traffic from donors who have recovered from COVID-19 and want to do something to help.

And it seems like they can -- thanks to some guinea pigs, and a procedure that is more than a century old. Thanks to watching this episode of SciShow, and thank you especially to our patrons. Your support is helping us keep people informed during a confusing and difficult time, and we are so, so grateful.

If you want to get involved, check out patreon.com/scishow. [♪ OUTRO].