Scientists have identified a previously unknown barrier that separates the bloodstream from smelling cells in the upper airway of mice. But this barrier ends up keeping some of the larger molecules of the body's immune system out, and that may be hindering the effectiveness of vaccines. The barrier, named the BOB, Blood-Olfactory Barrier, like Blood-Brain Barrier, might partially explain the prevalence of breakthrough COVID infections and why they are often associated with smell loss.