According to the Centers for Disease Control and Prevention, Americans who are moderately or severely immunocompromised may require a fourth dose of the COVID-19 vaccination at least six months after receiving the third dose.

According to the CDC’s amended instructions, those who are moderately or severely immunocompromised should have their third shot ‘immediately’ if more than 28 days have passed since receiving the second dose of the mRNA vaccine.

People in the same category’may’ receive another dose six months after receiving the third 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 those who are undergoing active cancer treatment for blood cancers or tumors.

They also include organ transplant and stem cell recipients, people with advanced HIV, and those taking high doses of corticosteroids or other immunosuppressive medications.

According to the CDC, there are approximately 9 million persons in the United States who are mildly to severely immunocompromised.

The agency does not refer the shots given to immunocompromised persons as “boosters” because it is considered that the initial two doses did not result in a robust immune response.

Federal health officials gave final approval for Americans to receive booster shots last week.

Certain people who received Pfizer vaccinations months ago are already eligible for a booster, and the CDC now says that certain Moderna and Johnson & Johnson recipients are as well.

In a larger shift, the agency is allowing people to ‘mix and match’ their extra doses regardless of which type they received first.