On Aug. 11, 2025, Researchers from the Translational Genomics Research Institute (TGen), part of City of Hope, and the Lundquist Institute for Biomedical Innovation at Harbor-UCLA Medical Center have identified a potential biomarker for long COVID. If the findings of their study are confirmed by other research centers, the biomarker could be the first specific and quantifiable indicator for confirming long COVID. Currently, clinicians confer a diagnosis of long COVID based upon a collection of symptoms that patients develop after SARS-CoV-2 infection.

The study results, reported in the journal Infection, detail the detection of SARS-CoV-2 protein fragments within extracellular vesicles (EVs) — tiny, naturally occurring packages that help cells share proteins, metabolites, and other materials. The researchers collected and analyzed blood samples from 14 patients over 12 weeks of aerobic exercise training (56 samples in all) in a clinical trial led by Stringer in long COVID.

The researchers found 65 distinct protein fragments from SARS-CoV-2 inside the EVs. These fragments come from the virus’s Pp1ab protein, an RNA Replicase enzyme which is key to how the virus copies itself and makes other viral particles. This protein is found uniquely in SARS-CoV-2, and not in uninfected human cells, noted Asghar Abbasi, Ph.D., a Lundquist Institute investigator and first author of the study.

Significantly, the researchers found that these viral peptides were demonstrated in each subject, but not each blood draw, in the EVs of Long COVID patients and were not detected in a separate control group of pre-pandemic EV samples.

These findings add to growing evidence that suggests that SARS-CoV-2 may persist in certain body tissues long after the initial infection. Some groups hypothesize these lingering viral reservoirs could play a role in Long COVID. How the virus reaches tissues without its usual entry points—such as the brain—remains an open question, and may be related to EV particles.