(WENY) – Currently, federal health experts have approved a third dose of Pfizer or Moderna for moderately to severely immunocompromised people; they also greenlit a booster shot for certain Pfizer recipients.

WENY News has compiled information breaking down what the difference between a third dose and a booster shot is, and who is eligible for which shot.

Third Dose vs. Booster Shot

According to Johns Hopkins Medicine, there is a difference between what is considered a “third dose” of an mRNA vaccine such as Pfizer or Moderna and what is considered a “booster shot”.

A third dose is given to those with seriously weakened immune systems who did not have a strong enough response to the first two doses of an mRNA vaccine to provide the same level of protection that most other Americans received. A third dose of a Moderna or Pfizer vaccine is given to eligible individuals as soon as 28 days after the second dose.

A booster shot is given to a larger group of people in order to boost protection after immunity provided from the original shots decreases over time. According to current guidance, the Pfizer booster is given to eligible people six months after their second dose. Pfizer is the only vaccine on the market that is approved for a booster shot, as of September 28.

Who’s Eligible?

Third Dose of Moderna or Pfizer 

Currently, the CDC is recommending that moderately to severely immunocompromised people receive an additional dose of either Pfizer or Moderna at least 28 days after their second shot. 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

Booster Shot of Pfizer 

On Friday, the federal government announced that certain groups of people that received the Pfizer vaccine should receive a booster shot six months after their second dose. This includes:

• People 65-years-old and older and adults 50-64 with underlying medical conditions should get a booster shot of Pfizer
• People aged 18-39 years with underlying medical conditions MAY get a booster shot based on their individual benefits and risks, which should be discussed with their doctor
• Residents 18 and older of long-term care settings
• People 18-64 who are at increased risk for COVID-19 exposure and transmission because of occupational or institutional settings MAY get a booster shot based on their individual benefits and risks. This includes people who work in healthcare, schools, correctional facilities, homeless shelters and grocery stores

The CDC currently lists the following underlying medical conditions as eligible for a booster shot:

• Having cancer or a history of cancer
• Chronic kidney disease
• Chronic lung diseases, such as COPD, asthma, interstitial lung disease, cystic fibrosis and pulmonary hypertension
• Dementia or other neurological conditions
• Type 1 or Type 2 diabetes
• Down syndrome
• Heart conditions such as heart failure, coronary artery disease, cardiomyopathies or hypertension
• Liver disease
• Obesity
• Pregnancy
• Sickle cell disease or thalassemia
• Smoking, current or former
• Cerebrovascular disease
• Substance use disorders

Again, booster shots are only approved for Pfizer vaccine recipients at this time. If you are unsure whether you should get a booster shot six months after your second Pfizer dose, or a third dose of Moderna or Pfizer, you should contact your doctor to determine what care is best for you.

This guidance is subject to change, and eligibility may expand as experts continue to study data surrounding the vaccines. Currently, there is no guidance for an additional dose or booster shot for Johnson and Johnson.