Blood transfusions are not currently recommended as a treatment option for COVID-19 patients.

Until the spring of 2021 in the United States, blood donations had been tested to see whether they contained COVID-19 antibodies. Now most hospitals have stopped testing blood and labeling the donations with this information. Originally, scientists were curious to see if blood (particularly a part of it called convalescent plasma) from recovered COVID-19 patents might help those who were actively fighting infections. After a study of tens of thousands of patients showed no clear benefits of using plasma from recovered patients to treat sick patients, this potential treatment was largely scrapped.

Antibodies are proteins that help our bodies fight off infections. When someone recovers from an infection they are often left with antibodies that help protect them against getting the infection again. Convalescent plasma (CCP) therapy involves using the antibodies in peoples' blood plasma to protect or treat other people. The therapy can be used in hospitalized patients with COVID-19, according to the U.S. FDA, which granted emergency use authorization to the treatment in August 2021.

Knowledge about the effects of this therapy as a COVID-19 treatment remains limited. Current evidence has not shown a significant benefit for high-risk, infected outpatients in early stages of infections in most studies. Research is currently ongoing to determine if the treatment may help prevent symptomatic COVID-19 after exposure, lessen the severity of symptoms, or shorten the length of the disease.

The United States National Institutes of Health recommends against the use of low-titer (lower number of antibodies) convalescent plasma for the treatment of COVID-19. Insufficient evidence exists to determine if high-titer convalescent plasma may be beneficial in COVID-19 patients.

In an FDA-sponsored program, scientists found that getting the therapy within three days of COVID-19 diagnosis was associated reduced mortality. Patients that were treated with more antibodies also seemed to have stronger responses to the therapy. However, patients were simultaneously being treated with other therapies too, so whether we can accurately credit CCP with these results is unclear.

However, in March 2021, researchers found that in patients with moderate or severe disease, CCP did not reduce mortality and had little to no impact on patients feeling better or worse.

As of August 2021, the U.S. National Institutes of Health’s (NIH) COVID-19 treatment guidelines recommend against the use of CCP in hospitalized patients who are not immunocompromised, unless they are enrolled in a clinical trial.

The Vitalant Research Institute in the United States estimates that "90% of current donors have either been infected with COVID-19 or vaccinated against it." Thus, if blood transfusions from recovered or vaccinated patients were shown to be beneficial in sick patients, it is highly likely this benefit would have been shown by now.

Similarly in South Africa, the presence of COVID-19 antibodies in the blood was seen in roughly 43% of donations across all provinces. In this case too, the potential impact on sick patients would likely have been seen by this point.

During the Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS) coronavirus outbreaks, findings from case studies suggested that CCP was safe and may offer benefits during the early stages of infectious diseases. Researchers proposed that early use of this treatment could blunt the impact of disease and the progression to severe disease. However, more research is needed to evaluate the effects of CCP therapy before health professionals can use it widely for COVID-19 treatment.