Although injected vaccines do reduce symptomatic COVID cases, and prevent a lot of severe illness, they may still allow for asymptomatic infection. A person might feel fine, but actually harbor the virus and be able to pass it on to others. The reason is that the coronavirus can temporarily take up residence in the mucosa—the moist, mucus-secreting surfaces of the nose and throat that serve as our first line of defense against inhaled viruses. Research with laboratory animals suggests that a coronavirus infection can linger in the nose even after it has been vanquished in the lungs. That means it might be possible to spread the coronavirus after vaccination.

Enter the intranasal vaccine, which abandons the needle and syringe for a spray container that looks more like a nasal decongestant. With a quick spritz up the nose, intranasal vaccines are designed to bolster immune defenses in the mucosa, triggering production of an antibody known as immunoglobulin A, which can block infection. This overwhelming response, called sterilizing immunity, reduces the chance that people will pass on the virus.