Provided that they received their primary vaccine series from either Moderna or Pfizer, adults with moderately or severely compromised immune systems are now eligible to receive a fourth dose six months after they received a third dose, according to the CDC. Immunocompromised individuals were given the green light by the Food and Drug Administration back in August to receive a third dose four weeks after getting their second.

The CDC said those with compromised immune systems can receive any authorized COVID-19 booster—Moderna, Pfizer, or Johnson & Johnson—as their fourth dose regardless of which vaccine they originally were given. People who receive Moderna for their fourth shot, though, will be given just a half dose (50 micrograms as compared with the usual 100 micrograms given for the first, second, and third Moderna jabs).

On the other hand, the CDC recommends that immunocompromised people who originally received the single-dose Johnson & Johnson vaccine get just one booster shot of any approved brand at least two months after receiving the primary dose. That is, in this case, immunocompromised individuals should receive just two total COVID-19 vaccine shots as opposed to four: the original Johnson & Johnson dose and the booster of any brand.

Immunocompromised individuals make up roughly 3% of the U.S. adult population, and they are both more vulnerable to COVID-19 and less likely to build the same level of immunity from a two-dose vaccine series as those who are not immunocompromised, according to studies cited by the CDC.

Other studies showed that fully vaccinated immunocompromised people made up roughly 44% of breakthrough cases that resulted in hospitalization.

Those who qualify as immunocompromised include active cancer patients, organ transplant recipients, people who have received stem cell transplants within the past two years, individuals with advanced or untreated HIV, and those taking drugs that may suppress the immune system, among others, according to a CDC list.

A fourth dose is currently not allowed for any other groups besides the immunocompromised.

Those who are at high-risk of COVID-19—individuals 65 and older as well as people ages 18 to 64 with underlying medical conditions or jobs or living situations that put them at high risk—should not receive a fourth dose as long as they have normal, functioning immune systems, according to the CDC. However, they are advised by the CDC and FDA to receive a booster shot six months after becoming fully vaccinated (two months if originally given J&J).

Currently, the CDC “does not have a recommendation for immunocompromised people” to receive a fourth dose and noted within this guidance that “a patient’s clinical team is best positioned to determine the appropriate timing of vaccination.”

Instead, the current CDC recommendation is for immunocompromised individuals to just receive a third dose 28 days after completing an mRNA vaccine series. Still, the updated guidelines give this group the option of receiving a fourth dose.

More health care and Big Pharma coverage from Fortune:

• Foreign travelers to the U.S. will need to be vaccinated and present a negative COVID test before entering starting Nov. 8
• Florida Gov. DeSantis offers $5,000 bonus to lure anti-vaxx police from out of state
• Who is eligible for a Moderna booster?
• Thera-who? These biotech firms are looking to push what’s possible with blood
• 3 states limit nursing home profits in bid to improve care