The US Centers for Disease Control and Prevention has approved the fourth dose of COVID vaccine for people with weaker immune systems.

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CDC this week updated its COVID-19 vaccine guidance to approve the fourth dose for immunocompromised people that includes organ transplant recipients, certain cancer patients, and people with HIV, at least six months after completing their primary vaccination series, NBC News reported.

In August, the CDC recommended that immunocompromised individuals who had been initially vaccinated with either the Pfizer or the Moderna mRNA vaccine -- but not Johnson and Johnson's vaccine -- receive a third dose of an mRNA vaccine. That extra dose was not considered a booster, but rather a part of their primary vaccination series.

The new booster dose can be any of the available vaccines, including the Johnson and Johnson vaccine, according to the CDC. Research has shown that people with weaker immune systems don't mount an adequate immune response following vaccination, and the purpose of the third dose was to raise their immunity levels to what's seen in people with normal immune systems after two doses.

The fourth dose aims to combat waning immunity while serving the same purpose as a booster dose given to people without immune deficiencies six months after they were initially vaccinated. "We know that six months after you reached a good level of protection, your protection has waned a and we need to boost that," Dr Dorry Segev, professor of surgery at Johns Hopkins University, was quoted as saying.

"That's for people with normal immune systems and people who are immunocompromised." However, not every immunocompromised American will need a fourth shot, the report said. "Out of the 11 million immunocompromised people in this country, some of them were fine with two doses," Segev said. "Some of them were not fine with three doses. Some of them do need a fourth dose."