Vaccines and tests were designed for explicitly different purposes. COVID-19 shots are proactive, forward-looking measures: They offer viral anatomy lessons to immune cells in advance of an encounter, schooling them on how to fend off a pathogen without actually forcing them to fight it. Infections of all severities are uncommon among the vaccinated. When they do occur, they’re milder, briefer, and less likely to spread to others, and they almost never end in hospitalization or death. Vaccines are an investment in the long term, a sustainable preventive against disease; they bolster the body’s defenses, upping a person’s chances of coming away from a viral encounter unscathed.

Tests, while powerful in their own right, don’t offer any of those perks. They’re measuring tools that home in on snippets of viral genetic material or hunks of viral proteins in someone’s airway, and can only identify infections that have already begun. That makes them reactive by default, Saskia Popescu, an infection-prevention expert at George Mason University, told me. Tests offer one result at a time, a snapshot—at the moment of sampling, you had a detectable smidge of virus in your nose or mouth, or you didn’t—with zero bearing on what’s to come. “You incur no protection from a test,” Susan Butler-Wu, a clinical microbiologist at the University of Southern California’s Keck School of Medicine, told me. “With a vaccine, you are protected, and you are protecting.”

The Biden administration, of course, is not billing tests as functional vaccines. But as written into the mandate, tests are still being cast in an understudy role that they were never meant to play. “It’s an unacceptable alternative,” Jennifer Nuzzo, an epidemiologist and senior scholar at the Johns Hopkins Center for Health Security, told me.