Explainer: What J&J's authorized COVID-19 shot means for the U.S. vaccination campaign

A vial of Johnson & Johnson's Janssen coronavirus disease (COVID-19) vaccine candidate is seen in an undated photograph. Johnson & Johnson/Handout via REUTERS.

NEW YORK (Reuters) - Johnson & Johnson's one-shot COVID-19 vaccine received U.S. authorization on Saturday and will start being administered to Americans within days.

The following is how it fits into the vaccination effort with the two other authorized immunizations from Pfizer Inc/BioNTech SE and Moderna Inc.