By IDSE News Staff

The final results of a clinical trial that investigated convalescent plasma did not show a significant benefit for outpatients with COVID-19 (N Engl J Med 2021 Aug 18. doi:10.1056/NEJMoa2103784).

The C3PO (COVID-19 Convalescent Plasma in Outpatients) trial demonstrated that COVID-19 convalescent plasma did not prevent disease progression in a high-risk group of outpatients with COVID-19, when administered within the first week of their symptoms. 

The trial was stopped in February due to lack of efficacy based on a planned interim analysis. 

Last year, the FDA issued an emergency use authorization to allow use of convalescent plasma in hospitalized patients with COVID-19. Researchers wanted to know whether administering COVID-19 convalescent plasma might also be beneficial in people who were recently infected with SARS-CoV-2, but who were not severely ill and could be treated as outpatients. 

The objective was to prevent progression to severe COVID-19 illness. The C3PO trial, launched in August 2020, was designed to answer that question. 

The randomized controlled clinical trial involved adult outpatients who presented to emergency departments (EDs) with mild COVID-19 symptoms during their first week post-infection. The trial was conducted by the Strategies to Innovate Emergency Care Clinical Trials Network, and enrolled more than 500 participants from 48 EDs across the United States. The participants were racially and ethnically diverse with a median age of 54 years, and slightly more than half were women. 

Participants also had at least one risk factor for progression to severe COVID-19, such as obesity, hypertension, diabetes, heart disease or chronic lung disease. The researchers randomly assigned the participants to receive treatment with either high-titer COVID-19 convalescent plasma (containing anti–COVID-19 antibodies) or placebo (salt solution infused with multivitamins and lacking antibodies).

Researchers compared outcomes in both groups within 15 days of treatment, looking specifically at whether the patients needed to seek further emergency or urgent care, were admitted to the hospital or died. The researchers found no significant difference in disease progression between the two groups. Of the 511 participants, disease progression occurred in 77 (30%) in the COVID-19 plasma group compared with 81 patients (31.9%) in the placebo group. 

The plasma intervention did not cause harm, the researchers found.

“We were hoping that the use of COVID-19 convalescent plasma would achieve at least a 10% reduction in disease progression in this group, but instead the reduction we observed was less than 2%," said Clifton Callaway, MD, PhD, the contact principal investigator for the C3PO trial and a professor of emergency medicine at the University of Pittsburgh. “That was surprising to us. As physicians, we wanted this to make a big difference in reducing severe illness and it did not.”

The reason the intervention did not produce the expected results is unclear, Dr. Callaway said. Researchers are continuing to look at possible explanations, including insufficient plasma dose, timing of plasma administration, host-related factors or other aspects of the host tissue responses to the infection, he added.

Additional studies of COVID-19 convalescent plasma are ongoing or planned in different populations. These included the Pass It On trial, a nationwide, National Institutes of Health–funded randomized clinical trial using convalescent plasma to treat hospitalized adult patients with COVID-19 infection to see whether the treatment can help them recover faster. Other trials include one in outpatients who are recovering at home, and one in individuals with a high risk for exposure to COVID-19 to see whether convalescent plasma can prevent infection.

“We need the results of these other convalescent plasma studies to get a clearer, more conclusive picture of its value for future treatments of COVID-19,” said Simone Glynn, MD, MPH, the chief of the NHLBI's Blood Epidemiology and Clinical Therapeutics Branch, who is coordinating the trial.

—The trial was funded primarily by the National Heart, Lung, and Blood Institute and by the Biomedical Advanced Research and Development Authority.