The Moderna and Pfizer vaccines are made with similar technology and provide similar high levels of protection against Covid. These vaccines require two doses several weeks apart.
The Johnson & Johnson vaccine is made with older technology and was designed to require only one dose. Recent studies suggest it is less effective against new strains of the coronavirus. Rare, but potentially serious, side effects have also been linked to this vaccine. For this reason, the Pfizer or Moderna vaccine is recommended for most people.