The Johnson & Johnson vaccine is an adenovirus vector vaccine. This means that it utilizes a dead, human adenovirus that contains the spike protein. When this virus attaches itself to a cell, the genetic material is recognized and an immune response is prompted.
This vaccine is similar to the mRNA Pfizer and Moderna vaccines as it also induces an immune response to the spike protein. However, it differs because it requires only one dose, can be stored in the refrigerator, and uses an adenovirus over mRNA.