Yesterday, the agency effectively acknowledged it had fallen behind the scientific evidence: Even though that evidence has not changed in months, the C.D.C. overhauled its guidelines. It said fully vaccinated people could stop wearing masks in most settings, including crowded indoor gatherings. The change sends a message: Vaccination means the end of the Covid crisis, for individuals and ultimately for society... Other experts worried that encouraging vaccinated people not to wear masks might cause unvaccinated people to shed them too — the so-called slippery-slope argument. It is a common concern whenever health authorities lift behavior restrictions. But history suggests it is often overblown. An absolutist message often fails, Julia Marcus of Harvard Medical School has noted, especially when it urges people to take steps that do not actually protect them.