Third COVID-19 vaccine doses for cancer patients: What to know

Last updated Oct. 25 2021

Research studies show that immunocompromised people who have low or no protection following two doses of mRNA COVID-19 vaccines may have an improved response after an additional dose of the same vaccine.

Based on this research, certain immunocompromised individuals, including certain cancer patients and survivors, can receive a third vaccine dose in order to strengthen their immune system’s response against COVID-19. But who is actually eligible for a third dose? And what can you expect if you or a loved one needs one?

To answer common questions about third COVID-19 vaccine doses, we spoke with infectious diseases specialist and head of Internal Medicine David Tweardy, M.D.

What is the difference between a third dose and a booster shot of COVID-19 vaccine?

An additional dose – also called a third dose – follows the initial two-dose vaccine series for people who may not have mounted a strong enough immune response after receiving the initial vaccine series. In other words, these individuals may not have had the same level of protection against COVID-19 as other individuals due to their weakened immune systems, so they need additional protection that can be provided by a third dose.

The FDA and Centers for Disease Control and Prevention (CDC) have authorized additional doses of Pfizer and Moderna’s COVID-19 vaccines for severely immunocompromised individuals.

By contrast, a booster dose is a supplemental dose given to groups whose immune response has waned over time.

Is the vaccine you receive for the third dose different than the vaccine people will receive for the booster?

According to the FDA, the same formulation is used for these vaccines. However, individuals that receive a Moderna booster receive a dosage half the size of the original dose; the Pfizer booster, on the other hand, is the same dose as the first two vaccines.

The Delta variant is highly transmissible and immunocompromised individuals may not have built up a strong immune response after the first two doses, it’s important to go ahead and get the third dose if you’re eligible. The hope is that will help strengthen your body’s immune response against COVID-19.

Who is eligible for a third dose of the COVID-19 vaccines?

People eligible for an additional COVID-19 vaccine dose include those who received the two-dose Moderna or Pfizer COVID-19 vaccine series and are moderately or severely immunocompromised, such as those with certain conditions identified by the CDC.

Examples include:

• Active or recent cancer treatment. Recent includes within one year of completing immunosuppressive therapy.
• Solid-organ transplant
• CAR T cell therapy 
• Autologous or allogeneic hematopoietic stem cell transplant within the last two years, or beyond two years for patients with chronic graft vs. host disease (GVHD), and/or ongoing immunosuppression
• History of primary immunodeficiency
• Advanced or untreated HIV infection
• People on active immunosuppressive therapy, such as chemotherapy, radiation therapy, high-dose corticosteroids, alkylating agents, antimetabolites, transplant-related immunosuppressive drugs, tumor-necrosis factor (TNF) blockers, certain biologic agents (e.g., rituximab) and other biologic agents that are immunosuppressive or immunomodulatory.

Do I need to get the same type of vaccine for my third dose as I got for the first two? For instance, if I got the Pfizer vaccine before, do I need to get the Pfizer vaccine again?

The CDC recommends that a third dose of the same mRNA vaccine be used, whenever available, for people who received either the Pfizer or Moderna COVID-19 vaccine series. If you’re not sure which mRNA vaccine you got or aren’t able to get the same vaccine, it’s OK to get either the Pfizer or Moderna mRNA COVID-19 vaccine.

What if I got the one-dose Johnson & Johnson vaccine? Do I need to another dose, or can I get the Pfizer or Moderna two-dose series?

The FDA has authorized a second dose of the Johnson & Johnson’s Janssen COVID-19 vaccine provides improved antibody response in immunocompromised people. The FDA’s authorization for additional doses only applies to the two mRNA COVID-19 vaccines. The FDA and CDC are working to provide guidance on this issue.

When should eligible cancer patients get a third dose?

People with moderately to severely compromised immune systems can receive an additional dose at least 28 days after their second dose of the Pfizer or Moderna COVID-19 vaccine.

However, patients currently receiving chemotherapy, immunotherapy, CAR T cells, stem cell transplants or participating in clinical trials may need to coordinate timing of their additional vaccine dose with their treatment schedule to optimize their vaccine response. Talk to your primary health care provider to determine the appropriate timing for your additional vaccination dose if you are immunocompromised or taking immunosuppressive therapies.

Can childhood cancer patients get a third dose?

Yes. Adolescents ages 12-17 years are eligible to receive the Pfizer COVID-19 vaccine. Those younger than age 18 are not authorized to receive the Moderna vaccine, and children younger than age 12 years are not authorized to receive any COVID-19 vaccine at this time.

What are the most common side effects of the third dose?

Reactions reported after the third mRNA dose are similar to what people experienced after receiving the two-dose series. The most commonly reported side effects are fatigue and pain at injection site. Most post-vaccination symptoms are mild to moderate and resolve within 1-2 days.

Do I need antibody testing to receive an additional vaccine dose?

No. Outside the context of research studies, the CDC does not recommend antibody testing to determine an immune response to vaccination. In many cases, you may have a negative antibody test result even if you are fully vaccinated.

What precautions do cancer patients need to take after receiving a third dose?

People who are immunocompromised may continue to experience a reduced immune response to COVID-19 vaccines, even after receiving the third dose. It is important to continue following COVID-19 safety precautions, including wearing a mask in indoor public settings, staying apart from others who are not from your household, and avoiding crowds and poorly ventilated indoor spaces. Because the delta variant is highly contagious, social distancing and wearing high-quality KF-94 or KN-95 mask may offer better protection.

It is also important to encourage your family, friends and anyone you spend time with to get vaccinated.

Will we need additional doses or boosters after this third dose?

This is something researchers are still looking at. They hope to gain a better understanding of whether future doses or boosters will be needed and, if so, how frequently.

Is MD Anderson offering the third dose of COVID-19 vaccines?

Yes. Eligible MD Anderson patients can sign into MyChart to schedule an appointment; you will need to bring your proof of prior vaccination and attest that you meet criteria for immunocompromised conditions. We are unable to accept walk-ins.

Appointments for community members who are eligible for a third dose of the vaccine will be available at MD Anderson soon.

Can I get the third dose somewhere else if it’s more convenient?

Yes. People who meet the eligibility criteria for additional COVID-19 doses should consider getting vaccinated wherever they can. Use the National Vaccine Finder to see appointments available in your community. Again, talk to your primary health care provider to determine the appropriate timing for your additional vaccination dose if you are immunocompromised or taking immunosuppressive therapies.

What is your advice to immunocompromised individuals who don’t want to get a third dose, either because they’ve already had COVID-19 or don’t want to get another shot?

Talk to your doctor about these concerns, but it’s really important to get the third dose if you are immunocompromised. The latest research indicates the COVID-19 vaccines are safe and effective, but their effectiveness may wane over time and/or they may become less effective with new variants. In addition, an immunocompromised individual may not have had a sufficient immune response to their initial vaccine series or infection; a third dose has been shown to improve immunity in those who have received a complete vaccine series, and it is expected to improve immunity in those who had a COVID-19 infection.