Plasma study hints how long Covid-19 antibodies last in the blood

This summer, hype over coronavirus antibodies peaked. Initially billed as a passport to freedom of movement and freedom to socialize, a less rosy picture has started to emerge.

Some reports suggest antibodies to the virus don't last forever, and instead fade over time. New research underscores the idea that antibodies aren't eternal, but at the same time, highlights the fact that, contrary to what you might think, this is not cause for alarm.

Instead, it is a call to action.

A research letter published last week in the journal Blood suggests blood levels of a specific type of antibody that binds to the coronavirus' spike protein wanes over time. The findings are based on samples from 15 blood-plasma donors who gave blood at least four times.

All 15 of these donors's blood saw declines in antibodies starting at 88 days post-symptom onset, but the most striking finding of this small study is that a further 21 days after 88 day mark, half of the antibodies in individuals' blood samples had vanished completely.

The findings suggest there may be a window of time in which a plasma donation for experimental Covid-19 therapies is more helpful. That treatment is convalescent plasma therapy, which is currently being tested out in a number of clinical trials.

"The best time to donate plasma would be the earliest time as possible after symptom resolution," says Renée Bazin, the study's first author.

What do declining antibodies really mean? — Fifteen people is a small sample, but the participants are part of a larger pool of 282 blood donors who gave blood to Héma-Québec, a non-profit organization, as part of a clinical trial. Bazin is director of innovation at Héma-Québec.

Within that larger group, 6.9 percent of people tested negative for coronavirus antibodies when they arrived to donate, despite definitely having had the virus. When the team only looked at samples donated between 11 and 12 weeks after symptom onset, the percentage of people without antibodies rose to 15 percent.

This added credence to the idea that coronavirus antibodies are not everlasting, but the decline doesn't spell bad news explains Bazin.

"It's not like you don't have antibodies anymore so you're in big danger or you won't be protected anymore," she tells Inverse.

The reason why is to do with the different types of antibodies. The main antibody researchers are interested in for coronavirus is called a "neutralizing antibody." It stops the virus from being able to enter a cell in the first place.

"We looked at total antibodies against the receptor binding site [the spike protein, which is used to enter cells]," she says. "When we look at neutralizing antibodies they decline even more rapidly."

Antibodies are produced by specific kinds of white blood cells within the body's adaptive immune system (the part of the immune system that can learn to fight a specific pathogen). This study suggests that, having beaten the virus, these cells eventually stop secreting new antibodies, while those already in the bloodstream begin to decline.

"What we see with SARS-CoV-2 is that there's a point in time where the pathogen is eliminated. And so there's no reason for the immune system to continue to produce these antibodies," she explains.

But aside from neutralizing antibodies, the immune system has other weapons in its arsenal. There are memory B-cells, a kind of white blood cell, which remember what pathogens look like and jump into action when they are encountered again. There's also a kind of immune cell called a T cell – these cells are trained to kill, destroying cells that become infected. Others, called helper T-cells, help B-cells produce antibodies.

Many studies and pre-prints (non-peer reviewed papers) are dedicated to elucidating the body's multi-pronged immune response to the virus. For example, a notable paper published in Cell found evidence of T cells in people who had recovered from Covid-19 — including those with mild symptoms.

"There are many studies right now looking at these memory responses," Bazin notes.

When studies find a decline of coronavirus antibodies, this result tells scientists nothing about what's happening with the other cellular fighters in the immune system, says Bazin. But where antibodies become critical post-fight is if they may be used as potential treatment for others.

What's up with convalescent plasma? –  In the United States, convalescent plasma therapy is at the center of a turbulent medical and political conversation about Covid-19 treatments. This came to a head in August, when the Food and Drug Administration issued an Emergency Use Authorization for convalescent plasma treatments, allowing the technique to be used to treat the severest of coronavirus cases.

The emergency approval was issued based on "signals of efficacy" from projects like the Mayo Clinic's Convalescent Plasma Expanded Access Program, but at the time, there had been no completed randomized controlled trials (two cited in the document were terminated early).

Since then, a September 16 trial (not controlled) from Houston Methodist Hospital found that of 25 severely ill individuals who received convalescent plasma, 19 showed improvement 14 days after transfusion. Eleven were eventually discharged. The study was published in The American Journal of Pathology.

Bazin cautions we still don't know for certain if the treatment works. And if it does work, it's not yet clear why.

"We don't know if it's beneficial. We don't know if these antibodies will be the active ingredient," she says. "We suppose that it will be, but we're not sure."

To answer those questions, clinical trials are ongoing. The American Red Cross and other organizations are calling for donations that may help answer these questions. If antibodies really are the key to beating Covid, this study suggests that there may be an expiration date on those blood samples.