(HealthNewsDigest.com) - An additional dose of a COVID-19 vaccine is recommended for people who are fully vaccinated and might not have had a strong enough immune response. In contrast, a booster dose is recommended for people whose immune response weakened over time.

The CDC recommends additional doses and booster doses of COVID-19 vaccines in specific instances:

• Additional dose. The CDC recommends a third dose of an mRNA COVID-19 vaccine for some people with weakened immune systems, such as those who have had an organ transplant. People with weakened immune systems might not develop enough immunity after vaccination with two doses of an mRNA COVID-19 vaccine. An additional dose might improve their protection against COVID-19.

  The third dose should be given at least 28 days after a second dose of an mRNA COVID-19 vaccine. The additional dose should be the same brand as the other two mRNA COVID-19 vaccine doses you were given. If the brand given isn’t known, either brand of mRNA COVID-19 vaccine can be given as a third dose.

• Booster dose. The CDC recommends a booster dose of the Pfizer-BioNTech COVID-19 vaccine for people age 65 and older, residents in long-term care settings and people ages 50 to 64 who have underlying medical conditions, such as cancer, diabetes and heart disease. A booster dose of the Pfizer-BioNTech COVID-19 vaccine is also available to people ages 18 to 49 with underlying medical conditions and people ages 18 to 64 who have a job that puts them at increased risk of being exposed to COVID-19.

  The booster dose should be given at least 6 months after the second dose of the Pfizer-BioNTech COVID-19 vaccine.

There isn’t enough research to determine if people with weakened immune systems who got a Janssen/Johnson & Johnson COVID-19 vaccine also have an improved response after getting an additional dose of the same vaccine. The CDC also hasn’t made recommendations about booster doses for people who got the Moderna or Janssen/Johnson & Johnson COVID-19 vaccine.