https://www.scientificamerican.com/article/to-beat-covid-we-may-need-a-good-shot-in-the-nose/

The development of highly effective COVID vaccines in less than a year is an extraordinary triumph of science. But several coronavirus variants have emerged that could at least partly evade the immune response induced by the vaccines. These variants should serve as a warning against complacency—and encourage us to explore a different type of vaccination, delivered as a spray in the nose. Intranasal vaccines could provide an additional degree of protection, and help reduce the spread of the virus.

The currently authorized vaccines are injected into the muscle of the upper arm. Through a variety of mechanisms, they simulate a coronavirus infection. To fight off this perceived attack, the immune system mobilizes antibodies and T cells. As a result, in the event of an actual coronavirus infection, the immune system is prepared with a strong defense.