How many of us have had COVID-19 or received vaccinations? The numbers of those inoculated are a bit easier to come by, but to count all those infected we’d have to test everybody. (That hasn’t happened – and won’t be happening.) Knowing the number of asymptomatic transmitters is helpful in both understanding who’s most impacted by COVID-19 and achieving the nirvana of herd immunity. A new study looks at a different population among us: blood donors.