The COVID-19 public health emergency has ended, but millions across the globe continue to deal with Long COVID. Researchers are still pursuing basic questions about Long COVID — its causes, how to test for it and how it progresses. Today, we look at a group of researchers studying the blood of some Long COVID patients in the hopes of finding a biomarker that could let physicians test for the disease.Questions? Thread of scientific research you're loving? Email us at shortwave@npr.org — we'd love to hear about it!