According to the U.S. Centers for Disease Control and Prevention, severely immunocompromised people may require a fourth mRNA coronavirus shot.

Though the CDC hasn’t officially recommended the fourth shot in its updated guidelines, the agency did say that people should talk to their doctors to see if the fourth shot is necessary.

According to the CDC, studies have shown that fully-vaccinated immunocompromised people make up 44% of the cases that led to hospitalization. A study from John Hopkins University found that fully vaccinated immunocompromised people are 485 times more likely than most vaccinated people to be hospitalized or die from COVID-19.

Moderately to severely immunocompromised people include those who are being treated for cancers of the blood or for tumors, certain organ transplant and stem cell recipients, people with advanced or untreated HIV, and those who take high-dose corticosteroids or other drugs that may suppress their immune systems, according to the CDC.

Roughly 9 million Americans, or 2% of the population, fall into the moderate to severe immunocompromised categories.

The CDC recommends people with conditions that compromise their immune systems should still try to avoid crowds and poorly ventilated spaces and should wear a mask in indoor public spaces.