The CDC's updated guidelines state those who are moderately and severely immunocompromised should "immediately" get their third shot if more than 28 days have passed since getting the second dose of the mRNA vaccine.
Six months after getting the third dose, people in that same category "may" get an additional dose.
"In such situations, people who are moderately and severely immunocompromised may receive a total of four COVID-19 vaccine doses," according to the CDC.
According to CNN, those who are moderately to severely immunocompromised are in active cancer treatment for cancers of the blood or tumors.
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They include organ transplant patients and stem cell recipients, people with advanced HIV, and those who receive high doses of corticosteroids or other immunosuppressants.
The CDC thinks roughly 9 million people living in the United States qualify as moderately to severely immunocompromised.
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The agency does not refer to shots given to immunocompromised people as "boosters" because it is believed they did not have a complete immune response from the initial two doses.
Last week, federal health officials gave ultimate approval for Americans to get booster shots.
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Certain people who got Pfizer vaccinations months ago are eligible for a booster, and now the CDC states specific Moderna and Johnson & Johnson recipients qualify, too.
It’s also important to note that now the agency allows the flexibility of "mixing and matching" that extra dose regardless of which type people got first.
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The Food and Drug Administration had already approved an expansion of the nation's booster campaign last Wednesday, and it was similarly endorsed Thursday by a CDC advisory panel.
CDC Director Dr. Rochelle Walensky had the closing word on who gets the extra doses.
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"These past 20 months have taught us many things, but mostly to have humility," she told the panel.
"We are constantly learning about this virus, growing the evidence base, and accumulating more data."
There still are limitations on who qualifies and when for a booster.
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Starting six months past their last Pfizer or Moderna vaccination, people are pressed to get a booster if they're 65 or older, nursing home residents, or at least 50, and have an increased risk of severe disease because of health problems.
Boosters were allowed, though not urged, for adults of any age at increased risk of infection because of health problems or their jobs or living conditions.
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