Starting later this week, some at-risk Americans become eligible for a 4th Covid shot.

For immunocompromised people who received a single shot of the coronavirus vaccine made by Johnson & Johnson, the C.D.C. does not recommend additional primary doses, but advises that they get a booster shot of the Moderna or Pfizer-BioNTech vaccines two months after the first dose.Some people are born with absent or faulty immune systems, and…