After a third dose of the Moderna and Pfizer vaccines was authorized for some immunocompromised people by the US Food and Drug Administration on Thursday night, the Centers for Disease Control and Prevention recommended that people with “moderately to severely” compromised immune systems get an extra dose of an mRNA vaccine.

“CDC does not recommend additional doses or booster shots for any other population at this time,” the agency said in its new guidance Friday.

People who meet the criteria can now get their third dose COVID-19 vaccine because the CDC has given its approval. A third shot is available to immunocompromised individuals who are recipients of solid organ transplants or have an “equivalent degree of immunocompromise” which means they have a lower ability to fight infections. This makes them more susceptible to the coronavirus. Although Booster authorization isn’t available to people with other chronic medical conditions or diseases, it might be. Dr. Fauci said, “We believe that sooner or later you will require a booster to ensure durability of protection.” Anthony Fauci spoke during a White House briefing on Thursday.

What does this all mean for the US? Read on for what we know about COVID-19 booster shots today, including why they’re needed, how they relate to breakthrough infections and what the controversy has been surrounding third shots. This page will be updated as more information becomes available.

Who is eligible for COVID booster shots

The CDC recommends that certain immunocompromised individuals receive an additional dose. This is a small group. The recommendation for this category is 18-years-old and older for Moderna and 12 years for Pfizer. Due to a lack data, the FDA did not authorize an additional dose of the Johnson & Johnson vaccine and the CDC didn’t recommend a second dose of the Pfizer vaccine for immunocompromised patients who received the one-shot vaccine.

About 3% of US adults are immunocompromised, according to the CDC, but research suggests they account for about 44% of hospitalized breakthrough cases of COVID-19. They are more likely to become very ill from COVID and have a lower immune response to vaccines.

People with heart disease or diabetes, such as those with type 2 and 3, are not advised to receive a booster. The CDC recommends that people with other conditions, such as heart disease or diabetes, get an additional dose (if they have the Moderna or Pfizer vaccine).

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Why are COVID booster shots so important?

Calling the eradication of the COVID-19 virus “unlikely,” a UK scientific advisory group says (PDF) there’s a “realistic possibility” that a variant will emerge that is resistant to the current battery of vaccines. All levels of development in coronavirus variants such as delta and lambda are being monitored by governments, public health agencies, and vaccine manufacturers. They hope to answer the question whether booster shots for new variants will soon be required among the general population.