The carefully orchestrated dance between the immune system and the viral proteins that induce immunity against COVID-19 may be more complex than previously thought. A new study by investigators at Brigham and Women’s Hospital used an ultrasensitive, single-molecule array (Simoa) assay to detect extremely low levels of molecules in the blood and measured how these levels change over the days and weeks following vaccination. The team found evidence of circulating protein subunits of SARS-CoV-2, followed by evidence of the body mounting its immune response and then clearing the viral protein to below the level of single-molecule detection. Results are published in Clinical Infectious Diseases.