FAQ: Booster vs Third Dose Eligibility

Booster or third dose?


  • Helps boost waning immunity
  • Given at least 6 months after initial vaccine schedule
  • For those 65+, long-term care residents, 18-64 with underlying conditions & 18+ at high risk of exposure
  • Pfizer only
  • Appointments available now

Third dose

  • Helps certain people reach a stronger level of immunity not reached after 2 doses
  • Given 28 days after the second vaccine dose
  • For those with certain immunocompromising conditions
  • Pfizer and Moderna
  • Appointments available now
 If you are currently eligible, schedule your appointment online


As the eligibility for the COVID-19 vaccine evolves, you may be wondering if you should get an additional dose. We answered some frequently asked questions below. 

What is the difference between a “third dose” and a “booster”?

A “third dose” of an mRNA vaccine (either Moderna or Pfizer) is available to people who may not have had a strong enough immune response after receiving the first two doses. For example, those who are immunocompromised need a third dose to reach a stronger level of immunity against the virus. 

A “booster” is an additional vaccine dose given to individuals who may have waning immunity because they completed their initial vaccine series many months ago. The rise of variants, such as the highly contagious Delta variant, is another reason why some people may be mounting a less robust immune response. 

Who is eligible for a third dose?

A third dose of the Pfizer or Moderna vaccine is available for people with certain immunocompromised conditions including current treatment for cancer, those who received an organ transplant or those with advanced or untreated HIV.

Learn more about the eligible conditions for third doses. Sign up here.

If you are eligible, you can schedule your third dose through our online scheduling portal. You will be asked to identify and confirm that you have one or more of the qualifying conditions when you schedule your appointment. If you have questions about whether you are immunocompromised, please speak to your treating clinician.

Who is eligible for a booster?

Booster doses of the Pfizer COVID-19 vaccine ONLY are now available for:

  • Those 65 and older
  • Those in long-term care settings
  • Those 18 to 64 who are at high risk of severe COVID-19
  • Those 18 and older at high risk of exposure to COVID-19 due to their work

If you are eligible, you can get your booster if it has been at least six months since your first two doses.

Learn more about eligibility for booster doses.

Sign up here.

I do not have any underlying conditions. Why would I need a booster?

It has been several months since the first round of COVID-19 vaccines were distributed and the available data shows people who got those doses of the vaccine may have waning immunity. A booster shot given approximately eight months after the initial vaccine series could potentially help boost protection against COVID-19.

Do I need to get the same vaccine for my additional shot or can I get another brand?

If possible, the additional dose should be the same type of vaccine that you initially received. However, the CDC states that you can get the other mRNA vaccine as your additional vaccine dose if the one for your original vaccine series is not readily available.

I am eligible for a third dose but received the single dose J&J/Janssen vaccine. Should I get a second dose?

No. The CDC and FDA have not yet determined whether you will likely have an improved immune response following an additional dose of J&J/Janssen vaccine. At this time, there also is no recommendation that recipients of the J&J/Janssen vaccine should get a dose of an mRNA vaccine.

Will I experience the same side effects for this extra dose as I did for my previous shots?

So far, reactions reported after the third dose were similar to that of the two-dose series: fatigue and pain at injection site were the most commonly reported side effects, and overall, most symptoms were mild to moderate. However, as with the two-dose series, serious side effects are rare, but may occur.

Booster Eligibility

Those with certain underlying medical conditions and those who are at an increased risk of exposure to COVID-19 due to their work are eligible to receive a booster dose of the Pfizer vaccine.

Underlying medical conditions include:

  • Chronic kidney disease
  • Chronic lung diseases (for example, COPD, moderate to severe asthma, interstitial lung disease, pulmonary hypertension, cystic fibrosis)
  • Dementia and other neurologic conditions
  • Diabetes (type 1 or 2)
  • Down syndrome
  • Heart conditions (for example, heart failure, coronary artery disease, and cardiomyopathy)
  • Hypertension
  • HIV infection
  • Immunocompromised state (unless have already received an additional dose as a result of your immunocompromised state)
  • Liver disease
  • Overweight and obesity (BMI > 25kg)
  • Pregnancy and recent pregnancy
  • Sickle cell disease
  • Smoking, current or former
  • Stroke or cerebrovascular disease
  • Substance use disorders

Those who are at increased risk for COVID-19 exposure include:

  • Healthcare workers
  • Unpaid caregivers of immunocompromised or frail persons
  • Law enforcement and other first responders
  • Workers who support education
  • Food and agriculture workers
  • Energy workers
  • Water and wastewater workers
  • Transportation workers
  • Public works
  • Workers who interact within <6 feet of others