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Boosters and Third Doses


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Updated November 16, 2021:

Certain groups can receive a booster or third dose of the COVID-19 vaccine.  This page offers clarification on who is eligible to receive an additional dose of vaccine and the difference between third doses and booster shots.

Ready to receive a booster or third dose? Click the button below to find your nearest vaccine site.


What is the Difference Between a 3rd Dose and a Booster?



Who qualifies for a booster dose?

In California, almost everyone is eligible to receive a booster dose.  You are eligible for a COVID-19 vaccine booster if:

  • 6 months has passed since your second dose of Pfizer or Moderna.
  • 2 months has passed since your first dose of Johnson & Johnson.

The State of California's eligibility process allows for individuals to self-determine their risk of exposure.

Marin County Public Health strongly recommends you get a booster dose as soon possible if you meet California's eligibility and you are:

  • Age 65 or older, OR
  • Age 18+ who have underlying medical conditions, OR
  • Age 18+ who live in long-term care settings, OR
  • Age 18+ who work or live in high-risk settings

How do I know if I have a qualifying underlying health condition?

Marin County Public Health uses the CDC's designations for identifying underlying health conditions. Examples of underlying health conditions include (but is not limited to) cancer, chronic kidney disease, diabetes, asthma, mental health diagnoses and obesity. However, the definition also extends to those that have been impacted by systemic health and social inequities, recognizing that inequality is also a contribution to negative health outcomes, and a booster dose may be beneficial to those impacted.

What defines a high-risk job?

In line with the CDC, high-risk jobs are defined as:

  • First responders (healthcare workers, firefighters, police, congregate care staff)
  • Education staff (teachers, support staff, daycare workers)
  • Food and agriculture workers
  • Manufacturing workers
  • Corrections workers
  • U.S. Postal Service workers
  • Public transit workers
  • Grocery store workers

Can I get a booster dose from a different manufacturer than my original vaccine?

Yes, state and federal vaccine regulatory agencies have approved use of the vaccines as a “mix and match” booster option. This means your single booster dose could be a different brand than your original vaccine series. Eligibility and booster dose timing is based on the brand of your original vaccine series. For example,

  • If you originally received Johnson & Johnson, you may receive a Pfizer, Moderna (half dose), or Johnson & Johnson booster vaccine two months after your original dose.
  • If you originally received Moderna or Pfizer and you meet eligibility for a booster (age 65 & older; age 18 & older with underlying medical conditions; OR age 18 & older and live/work in a high-risk setting), you may receive a Pfizer, Moderna (half dose) or Johnson & Johnson booster vaccine six months after completing your original vaccine series. 


Who qualifies for a third dose?

A third dose of the Pfizer or Moderna COVID-19 vaccine is recommended by CDC for severely immunocompromised people who received their second dose at least 28 days ago, and 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 medications that may suppress your immune response

Why are third doses limited to such a small group?

The CDC estimates that about 2.7% of the population of the United States is immunocompromised, including organ transplant recipients, people undergoing cancer treatment, and those with other immunodeficiencies. In Marin County, this represents approximately 10,000 people. This population is more likely to become infected with COVID-19, even after vaccination, compared with similarly vaccinated but otherwise healthy individuals. They are also more likely to become severely ill due to COVID-19.


Where can I access a booster or third dose?

Start with your health care provider. Your doctor will know whether you will benefit from a booster shot or third dose. Providers who offer regular vaccinations (e.g., flu, pneumovax, shingles) should provide eligible patients with COVID-19 booster vaccines as capacity allows.

Most vaccine providers in Marin County are providing third doses and boosters to people who are eligible. Vaccine providers include:

Due to high demand , appointments are required by most providers for third doses and booster shots. Not all providers offer drop-in vaccinations: walk-ups are available on a very limited basis as supply allows, and may not be available at all locations. COVID-19 vaccine brand (e.g., Moderna, Pfizer, Johnson & Johnson) availability will vary by location. Verify that a provider offers the booster or third dose brand that you are seeking prior to walking-in or making an appointment.  Online screening tools for retail pharmacies may not immediately reflect California's new booster dose eligibility: consider a walk-in appointment or seeking a vaccine from another location.



More Information and Frequently Asked Questions

Stay up-to-date with the latest information about booster and third doses by following the latest information on the California Department of Public Health (CDPH) and CDC websites, and Marin County Public Health (MCPH) COVID-19 Booster FAQs .