(CNN) -- People with certain health conditions that make them moderately or severely immunocompromised may get a fourth mRNA Covid-19 shot, according to updated guidelines from the US Centers for Disease Control and Prevention.
The CDC authorized a third dose for certain immunocompromised people 18 and older in August. It said a third dose, rather than a booster -- the CDC makes a distinction between the two -- was necessary because the immunocompromised may not have had a complete immune response from the first two doses.
A study from Johns Hopkins University this summer showed that vaccinated immunocompromised people were 485 times more likely to end up in the hospital or die from Covid-19 compared to most vaccinated people. In small studies, the CDC said, fully vaccinated immunocompromised people accounted for about 44% of the breakthrough cases that required hospitalization. People who are immunocompromised are also more likely to transmit the virus to people who had close contact with them.
The US Food and Drug Administration has also authorized booster shots of all three available vaccines for certain people and that would include the immune-compromised, the CDC says.
Research showed that a booster dose enhanced the antibody response to the vaccine in certain immunocompromised people.
That would make for a fourth shot at least six months after completing the third mRNA vaccine dose. At this time, the CDC does not have a recommendation about the fourth shot. People should talk to their doctors to determine if it is necessary, the CDC says.
Moderately to severely immunocompromised people include those who are in active cancer treatment 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 a high-dose corticosteroids or other drugs that may suppress their immune systems. The CDC estimates about 9 million people who live in the US, or about 2% of the population, fall into this category.
People who are immunocompromised who got the single-dose Johnson & Johnson shot should get a booster at least two months after their initial vaccine. People who choose a Moderna vaccine as a booster, even if they received a different vaccine as the first dose, should get the half-dose sized shot that was authorized as a booster for Moderna's vaccine, the CDC said.
Even if they are vaccinated, 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.
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