Unprecedented nationwide blood studies seek to track U.S. coronavirus spread
We still don’t know how many people have been infected with the novel coronavirus, SARS-CoV-2. Not only have countries struggled to roll out wide-scale testing for the virus, those efforts inevitably will miss people who have recovered from an infection. The best way to figure out how far and wide the virus has spread in a population is to look at blood. Antibodies, blood proteins that the immune system produces to attack pathogens, are viral fingerprints that remain long after infections are cleared. Sensitive tests can detect them even in people who never felt a single symptom of COVID-19.