A third dose is specifically for individuals who are moderately to severely immunocompromised and may not have built up an adequate protective immune response with their first series of vaccinations. Currently, the only authorized third doses are for the Moderna and Pfizer Comirnaty vaccines.
The recommendation is for immunocompromised individuals to receive a third dose of the same mRNA vaccine at least 28 days after receiving the second dose in the series (for individuals 18 years and older for Moderna and 12 years and older for Pfizer Comirnaty).
At this time, the CDC does not have enough data to suggest an additional dose of the Johnson & Johnson vaccine would produce an enhanced immune response in an immunocompromised person.
A booster dose is for people whose immunity may be fading after they complete their first vaccination series. There is currently no COVID-19 vaccine authorized as a booster.
Malheur County Health Department 
Thank you for the information on the third/booster vaccination. Question is how do you determine if a person’s immune system has been compromised? Stu Edwards
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Currently, CDC is recommending that moderately to severely immunocompromised people receive an additional dose. This includes people who have:
Been receiving active cancer treatment for tumors or cancers of the blood
Received an organ transplant and are taking medicine to suppress the immune system
Received a stem cell transplant within the last 2 years or are taking medicine to suppress the immune system
Moderate or severe primary immunodeficiency (such as DiGeorge syndrome, Wiskott-Aldrich syndrome)
Advanced or untreated HIV infection
Active treatment with high-dose corticosteroids or other drugs that may suppress your immune response
People should talk to their healthcare provider about their medical condition, and whether getting an additional dose is appropriate for them.
Hi, Stu – the graphic on our latest post might help with that. People who have received cancer treatments, are on medicine to suppress the immune system, etc.. https://malheurhealth.org/2021/09/13/testing-and-vaccine-clinic-tuesday-at-fairground/