COVID-19 vaccines: boosters + third dose recommendations

senior woman show red heart shape with syringe icon, after vaccinated or inoculation booster dose due to spread of corona virus, population, social or herd immunity concept

COVID-19 vaccines are widely available in the U.S. and have been protecting people from getting sick or experiencing severe symptoms of infection for almost a year now. As of Oct. 23, more than 414 million COVID-19 vaccine doses have been administered in the U.S. since last December.

But as the virus evolved and the Delta variant began to surge earlier this year, the data began to show the need for a vaccine booster – or third dose – for certain people, to help keep us protected. This is common for many vaccines and largely depends on the type of vaccine and how the virus it protects us against evolves.

Here’s what to know about COVID-19 vaccine boosters and additional doses.

COVID-19 booster vs. additional dose

Boosters and additional vaccine doses aren’t the same and the recommendations for COVID-19 vaccines are different at this time.

  • Additional doses are for people who didn’t build enough protection from the initial COVID-19 vaccine series, and
  • Boosters, typically a lesser dose than the initial series, are for fully vaccinated people whose immune systems already built an appropriate level of protection but over time, this protection may need to be “boosted” again.

Additional dose recommendation

In the case of COVID-19 mRNA vaccines (Pfizer/Comirnaty and Moderna), the additional (third) dose is the same dosage as the initial 2-dose series. No one should receive more than 3 doses.

  • Who needs an additional mRNA dose: Only those who are moderately to severely immunocompromised. Talk to your doctor to discuss if you think this applies to you.
  • When: 28 days after the second dose in the series.
  • Why: Because an immunocompromised individual may not build the same level of immunity to the 2-dose vaccine series as others, a third dose can improve the response to the initial series.
  • Can you mix and match brands? A third dose of the same mRNA brand initially received is recommended, but if the mRNA brand initially received is unavailable, either mRNA brand can be taken for an additional dose.
  • What about the one-dose J&J vaccine? According to the CDC, there is not enough data at this time to determine whether immunocompromised people who received J&J/Janssen vaccine also have an improved antibody response following an additional dose of the same vaccine.

Booster recommendation

As of Oct. 21, both the CDC and FDA now recommends certain people get a booster shot for any of the 3 approved or authorized COVID-19 vaccines in the U.S.

  • Who needs it:
  • Why: Studies show that after getting vaccinated against COVID-19, protection against the virus may decrease over time and be less able to protect against the Delta variant. Many older adults and people working in high-risk settings were some of the first vaccinated (as early as December 2020) so possible waning immunity was considered.
  • When:
    • Pfizer/Comirnaty and Moderna: 6 months after the second dose in the series.
    • J&J/Janssen: 2 or more months after the initial vaccination.
  • Can you mix and match brands? If the vaccine brand initially received is unavailable, any brand can be taken for the booster.

Data from the National Institutes of Health shows that the mix-and-match approach is safe,” says Dr. Suzanne Corrington, medical director for BlueCross BlueShield of Tennessee. “This approach could also make enhanced protection more readily available, regardless of which vaccine you initially received.”

Need more advice?

Speak to a provider who knows your medical history if you have questions or concerns about vaccines based on your health status. Your friends and family may have good intentions, but they may not know your body like you and your doctor, so it’s important to speak to a provider who knows you well.

If you do decide to go online to learn more about vaccines, do seek reputable sources like the CDCFDA or World Health Organization (WHO). BlueCross BlueShield of Tennessee members can also visit to get the latest on our support for COVID-19.

More COVID-19 vaccine information from BlueCross medical experts


Ali Whittier, CHES®

Ali joined the BlueCross BlueShield of Tennessee corporate communications team in 2014 and is a Certified Health Education Specialist (CHES®) through the National Commission for Health Education Credentialing (NCHEC). A native of Iowa, she has more than a decade of experience in health promotion and community engagement, as well as health care communications. When she’s not at BlueCross, she and her husband Spencer are racing their bikes, spending time outdoors or cooking healthy food and treats in their kitchen.

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