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Pfizer, Moderna COVID-19 Vaccines Less Effective in Immunocompromised People, But Help Avoid Hospitalization

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The Pfizer-BioNTech and Moderna COVID-19 vaccines help keep people with weakened immune systems out of the hospital if they are infected, but don’t offer as much protection as they do in people with fully functioning immune systems, according to a study.

The results support immunocompromised people receiving three doses of either vaccine, plus a fourth shot as a booster, the researchers said.

The study was published online on Nov. 2, 2021, by Morbidity and Mortality Weekly Report. Read “Effectiveness of 2-Dose Vaccination with mRNA COVID-19 Vaccines Against COVID-19–Associated Hospitalizations Among Immunocompromised Adults — Nine States, January–September 2021.”

About the Pfizer and Moderna COVID vaccines
About the study
What this means for you 

About the Pfizer and Moderna COVID vaccines

The Pfizer-BioNTech and Moderna vaccines work by using genetic material called messenger RNA (mRNA). The mRNA vaccines contain a small piece of the coronavirus’s mRNA, which tells your body to make copies of a protein in the virus called the spike protein. Your immune system then builds up immune cells and special proteins (antibodies) to fight the spike protein. So, if you’re ever exposed to the COVID-19 virus, your immune system is ready to recognize the virus and protect you from infection.

Because mRNA is just a small piece of the virus copied in a lab, the vaccine cannot cause COVID-19, and it is considered safe for immunocompromised people. Still, earlier studies suggest these vaccines may offer less protection against COVID-19 in people with weakened immune systems.

In August 2021, the U.S. Food and Drug Administration (FDA) granted full approval to the Pfizer-BioNTech COVID-19 vaccine. The vaccine is now marketed as Comirnaty and can be used in people ages 5 and older.

The Moderna vaccine remains authorized for emergency use in people ages 18 and older.

The initial doses of the vaccines are given as two separate injections in the upper arm muscle. The Pfizer vaccine doses are given three weeks apart, and the Moderna doses are given one month apart.

The U.S. Centers for Disease Control and Prevention (CDC) recommends that certain people, including immunocompromised people and people diagnosed with cancer, who were fully vaccinated with either the Pfizer or Moderna vaccines get a booster shot — a third vaccine dose — no sooner than 28 days and no later than six months after their second shot.

In October 2021, the CDC said that people with moderately or severely compromised immune systems were eligible for a fourth dose — a second booster shot for a total of four shots — of either the Pfizer or Moderna vaccines.

Experts have recommended that most people who have been diagnosed with cancer or have a history of cancer should get a COVID-19 vaccine. Still, you should talk to your doctor about whether getting vaccinated is the right decision for your individual situation.

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About the study

Earlier research has suggested that immunocompromised people might not develop the same number of antibodies against COVID-19 as people with fully functioning immune systems after being vaccinated. This means immunocompromised people would have less protection against being infected with COVID-19.

The researchers who did this study wanted to see how effective the Pfizer and Moderna vaccines were in keeping immunocompromised people who got infected with COVID-19 out of the hospital.

The researchers looked at information on adults hospitalized for COVID-like illness from 187 hospitals in nine states from Jan. 17, 2021 to Sept. 5, 2021.

The study included information on:

  • 20,101 immunocompromised adults; 53% had received two doses of either the Pfizer or Moderna vaccines
  • 69,116 adults with fully functioning immune systems; 43% had received two doses of either the Pfizer or Moderna vaccines

Among the immunocompromised people:

  • half the people were older than 70 and half were younger
  • 41% had received the Moderna vaccine
  • 59% had received the Pfizer vaccine
  • 44% had a solid-tumor cancer, which means the cancer has no cysts or liquid areas
  • 14% had blood cancer
  • 25% had a rheumatologic or inflammatory disorder, such as rheumatoid arthritis, psoriatic arthritis, or lupus
  • 7% had received organ or stem cell transplants
  • 32% had some other type of immune condition or immunodeficiency

Among the people with fully functioning immune systems:

  • half the people were older than 68 and half were younger
  • 42% had received the Moderna vaccine
  • 58% had received the Pfizer vaccine

Overall, the results showed that the vaccines were 77% effective at keeping immunocompromised people from being hospitalized with COVID and 90% effective at keeping people with fully functioning immune systems from being hospitalized with COVID.

This difference was the same no matter the vaccine brand, person’s age, or COVID-19 variant.

Specifically in immunocompromised people:

  • the Moderna vaccine was 81% effective
  • the Pfizer vaccine was 71% effective

The effectiveness of the vaccines did vary depending on why a person was immunocompromised. Vaccine effectiveness was:

  • 59% for people who had organ or stem cells transplants
  • 79% for people with a solid-tumor cancer
  • 74% for people with blood cancer
  • 81% for people with rheumatologic and inflammatory conditions

“Immunocompromised persons benefit from and should receive COVID-19 vaccines,” the researchers wrote. “Given that [vaccine effectiveness] is lower compared to immunocompetent patients, immunocompromised persons receiving mRNA vaccines should receive three doses and a booster 6 months after the third dose, consistent with CDC recommendations. In addition to vaccination, immunocompromised persons should implement nonpharmaceutical prevention strategies such as masking to help prevent SARS-CoV-2 infection, and, if infected with SARS-CoV-2, be monitored closely and considered early for proven therapies that might prevent progression to severe illness.”

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What this means for you

If you’ve been diagnosed with breast cancer and are wondering how well the Pfizer or Moderna vaccines may work for you, this study offers important results.

While the vaccines were not 100% effective, it’s very encouraging to know that they were more than 75% effective at keeping immunocompromised people from being hospitalized with COVID.

The results also strongly suggest that getting a second booster shot — a fourth dose of any COVID-19 vaccine — may be a good idea if you’ve been diagnosed with breast cancer.

Current recommendations say that immunocompromised people ages 18 and older should receive:

  • a third full dose of either the Pfizer or Moderna vaccine 28 days after receiving their second shot
  • a fourth dose of the Pfizer, Moderna, or Johnson & Johnson vaccine at least 6 months after receiving their third shot; if your fourth dose is the Moderna vaccine, it should be a half dose

Still, each person’s health situation is unique. So it makes sense to talk to your doctor about whether a COVID-19 vaccine and booster shots are right for you.

Read more about COVID-19 Vaccine Facts for People With Breast Cancer.

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Written by: Jamie DePolo, senior editor


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