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

The CDC, which updated its guidelines Tuesday, said people who are moderately and severely immunocompromised may need the additional shot.

“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,” the health agency wrote on its website.

“In such situations, people who are moderately and severely immunocompromised may receive a total of four COVID-19 vaccine doses,” the CDC continued.

The CDC classifies the third vaccine shot for immunocompromised people as an “additional dose” at the same volume given in the first two shots. The fourth dose is considered a “booster” for those individuals, and administered at a volume of only half that was given in the first three doses.

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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 six months or more after their initial series:

• 65 years and older
• Age 18+ who live in long-term care settings
• Age 18+ who have underlying medical conditions
• Age 18+ who work or live in high-risk settings

The CDC now says booster shots are recommended for those who received the single-dose Johnson & Johnson COVID-19 vaccine, too. 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 by visiting vaccines.gov.