The CDC's updated guidelines state that those who are moderately and severely immunocompromised should 'immediately' receive their third shot if more than 28 days has elapsed since receiving the second dose of the mRNA vaccine.

Six months after receiving the third dose, people in that same category 'may' receive another 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.

Those who are moderately to severely immunocompromised are those who are in active cancer treatment for cancers of the blood or for tumors, according to CNN.

They also include organ transplant patients and stem cell recipients, people with advanced HIV, and those who take high doses of corticosteroids or other immunosuppressants.

The CDC believes that there are about 9 million people living in the United States who qualify as moderately to severely immunocompromised.

The agency does not refer to shots given to immunocompromised people as 'boosters' because it is believed they did not have a full immune response from the first two doses. 

A study by Johns Hopkins University found that vaccinated immunocompromised people were 485 times more likely to be hospitalized or die from COVID-19 compared to most vaccinated people.

Those who are immunocompromised are also more likely to transmit the virus to people who had close contact with them.