Experts say the protection that vaccines offer is more like a coat of flame retardant than an impenetrable firewall. SARS-CoV-2 can, very rarely, still set up shop in people who are more than two weeks out from their last COVID-19 shot. Chasing constant reassurance about infections after vaccination could send the wrong message, according to Saskia Popescu, an infection-prevention expert at the University of Arizona. "If we're telling people they can be unmasked and we’re still (frequently) testing them, what's the signal we're sending?"