One shot instead of two
The Johnson & Johnson vaccine is administered in a single shot, while the Pfizer-BioNTech and Moderna vaccines are given in two shots several weeks apart.
The way it works
The Johnson & Johnson vaccine uses a different method to prime the body to fight off Covid-19: a viral vector called Ad26. Viral vectors are common viruses that have been genetically altered so that they do not cause illness but can still cause the immune system to build up its defenses. The Pfizer-BioNTech and Moderna vaccines use messenger RNA to do that.
How well it works
The Johnson & Johnson vaccine is rated as highly effective at preventing serious illness and death, as the Pfizer-BioNTech and Moderna vaccines are. It is also very effective at preventing milder illness, though a bit less so than those two. It appears to do well against the highly contagious B.1.351 variant, first identified in South Africa, that has given at least one other vaccine candidate trouble.