Johnson & Johnson's COVID-19 vaccine is likely to be approved soon. This vaccine is like the Pfizer and Moderna vaccines in that it presents genetic instructions to human cells to make coronavirus spike protein. Cells then make and release spike protein, which the immune system detects as foreign, attacks, and remembers. If a vaccinated person is later exposed to SARS-CoV-2, the virus that causes COVID-19, the immune system disables the spike protein upon contact, which prevents the virus from entering cells, replicating, and causing disease. The J&J vaccine differs from the first two in that the genetic information comes in the form of double-stranded DNA packaged in an adenovirus (unrelated to coronaviruses) which has been modified so that it can enter cells but not reproduce. It further differs in that the DNA enters the cell nucleus, where its genetic information is copied into messenger RNA (mRNA). The mRNA returns to the outer part of the cell, where it directs the cell to make spike protein. (The AstraZeneca COVID-19 vaccine works the same way.)
In contrast, the Pfizer and Moderna vaccines consist of single-stranded mRNA packaged in ultrafine fat bubbles which are taken up by cells. Without entering the cell nucleus, the mRNA directly instructs the cell to make spike protein, after which the cell chops up and disposes of that mRNA. The Pfizer and Moderna vaccines require two doses; the J&J vaccine requires only one dose and is easier to store. All three have been proven to be highly effective and safe. The best one to get is whichever one you can get soonest!