(WJW) — As some people receive a third dose, or booster shot, of the COVID vaccine, the Centers for Disease Control and Prevention are now talking about a fourth dose.

The CDC said people who are moderately and severely immunocompromised may need it.

The health agency wrote on its website:

“Moderately and severely immunocompromised people aged ≥18 years who completed an mRNA COVID-19 vaccine primary series and received an additional mRNA vaccine dose may receive a single COVID-19 booster dose (Pfizer-BioNTech, Moderna or Janssen) at least 6 months after completing their third mRNA vaccine dose. In such situations, people who are moderately and severely immunocompromised may receive a total of four COVID-19 vaccine doses.”

When it comes to booster shots, for those who have received a Pfizer-BioNTech or Moderna COVID-19 vaccine, the CDC says the following groups are eligible for a booster shot at 6 months or more after their initial series:

The CDC also now says booster shots are recommended for those who received the Johnson & Johnson COVID-19 vaccine. In this case, the recommendation is for anyone 18 and older, who was vaccinated two or more months ago.

You can find a COVID vaccine near you, right here.