The Johnson & Johnson (Janssen) vaccine is a single-dose vaccine. It’s different than the Pfizer or Moderna vaccine because it’s a viral vector vaccine. It uses use a modified version of a different virus (the vector) to deliver important instructions to our cells. For COVID-19 viral vector vaccines, the vector (not the virus that causes COVID-19, but a different, harmless virus) will enter a cell in our body and then use the cell’s machinery to produce a harmless piece of the virus that causes COVID-19. Experts began creating viral vectors in the 1970s. The vaccine will have been studied in more than 45,000 participants and passed the same safety standards as other vaccines before it is approved.