The Food and Drug Administration on Thursday approved extra doses of coronavirus vaccine for immunocompromised people in the United States, and the Centers for Disease Control and Prevention followed up with its own approval on Friday.

The decision will provide millions of Americans who may have a less-powerful response to coronavirus vaccines an option to boost their bodies’ defenses against covid-19, the disease caused by the virus, as the more contagious delta variant drives up infections. Health officials have said they see the extra shots as part of the primary series for such people.

The move could mean additional shots will be available for this vulnerable population as soon as this weekend.

Here’s what you need to know.

What to know

  • What does it mean to be immunocompromised?
  • What conditions can compromise the immune system?
  • How does this relate to the coronavirus?
  • How does the United States compare with other countries?
  • Have we learned lessons from these countries’ experiences?
  • What does the announcement mean for people who are not immunocompromised?