"We are aware that this population may not have the same beneficial response to vaccines that we see in others who are not immunosuppressed," Dr. Linda Finn, the director of hematology and bone marrow transplant at Ochsner Health, told ABC News. "We do still recommend the vaccines, any benefit they can borrow from it, but still follow other precautions very closely."
Although clinical trials found vaccines were highly effective in preventing symptomatic or severe COVID-19 infections, these trials largely excluded people who were immunocompromised.
That means these estimates may not apply to people with weakened immune systems, such as people with cancer or HIV.
It can also include people who are taking medications that weaken the immune system to manage medical conditions, like those with autoimmune conditions or who have had transplants. Approximately 3% of the U.S. population is immunocompromised, according to the Centers for Disease Control and Prevention’s National Health Interview Survey.
Recent studies suggest that some of the conditions or treatments for the conditions interfere with the body's ability to create antibodies that help protect against COVID-19 after vaccination.
"It's been disheartening, obviously, for the patients to begin to realize, 'Hmm, this number of 95% effective that you hear about in the mRNA vaccine trials does not necessarily apply to me,'" Dr. Ghady Haidar, a transplant infectious disease physician at the University of Pittsburgh Medical Center, told ABC News.
Normally a fully vaccinated person will produce COVID-19 antibodies, and those antibodies should show up on an antibody test. But some fully vaccinated people who are immunocompromised have been surprised to learn they're not producing enough antibodies to test positive on an antibody test.
In one study, just over half of patients with blood, bone marrow, and lymph node cancer had antibodies; in another, less than 40% of patients with the blood cancer CLL (chronic lymphocytic leukemia) had an antibody response.
It is not yet known why this may be happening. The reduced antibody response may be due to these patients' underlying diseases, the medications they are taking, or both. Additional studies are needed to figure this out.
Experts think the vaccine should provide some protection for those who are immunocompromised, but they are still working to determine how much.
Researchers noted that antibody monitoring only measures one way the body responds to infections and there are other ways your immune system might be protecting you that wouldn't show up on an antibody test, including T-cell response, which is more challenging to test for than antibodies.
"It's definitely possible that there was still immunity built up even if the test didn't acknowledge the presence of antibodies," said Dr. John Brownstein, an epidemiologist at Boston Children's Hospital and an ABC News contributor.
"We need to develop protocols for those who are antibody negative [after vaccination]," said Dr. Mounzer Agha, a hematologist at UPMC Hillman Cancer Center and the lead author of the first study mentioned above. "The reason for testing right now would be to tell patients that they may still be vulnerable and should continue to exercise caution."
"No matter what the antibody results are, that should be the message: Continue living life safely, masking and social distancing," Haidar added.
Experts agreed that a patient's immunocompromised status is not a reason to not get a COVID-19 vaccine and that it is not only safe to do so, but very important, as they could be more at risk of becoming severely ill from COVID.
"Unless there is an absolute contraindication, which may be some allergy guidelines to avoid reactions to prior vaccines, I haven't had a patient yet that I have not advised to get the vaccine if they can," said Finn.
People with weakened immune systems will also be protected if everyone in their surrounding community is fully vaccinated, which is a concept known as "herd immunity."
Finn is recommending people get vaccinated with "what is available in their area." There is no data that favors one type or brand of vaccination over another for people with these conditions.
Experts agree that more data is needed to understand when to give vaccines to people undergoing chemotherapy, and those taking immunosuppressive medications. For now, the CDC recommends that patients starting immunosuppressive therapies get vaccinated at least two weeks prior.
Finn said it's a bit more nuanced and depends on your personal treatment plan. While she generally agrees with following the CDC's recommendations, this isn't always possible, as some patients need tighter treatment schedules with more individualized discussions.
Patients should talk to their doctors about what to do with their immunosuppressive medications and the timing of vaccination, as it may not be safe to space out, pause, or stop their medications.
Even after vaccination, people who are immunocompromised should continue to take extra precautions to avoid getting COVID-19.
"I think this patient population should expect to be following pretty broad COVID precaution guidelines for a long time," said Finn.
Some people may not know if they have a condition or take a medication that may impact the effectiveness of the vaccine. This is another reason continuing to mitigate risk with masking in crowded or indoor locations is recommended.
Haidar notes that immunocompromised people can also "encourage others around them to live their lives safely, including getting vaccinated," a step which benefits the individual and those around them.
Amanda Benarroch, M.D., is a psychiatry resident at Mayo Clinic in Rochester, Minnesota. Olivia Davies is a fourth-year student at the Medical College of Wisconsin who will be starting her residency at Massachusetts General Hospital this summer. Karine Tawagi, M.D., is a hematology and oncology fellow at Ochsner Medical Center in New Orleans. All three are contributors to the ABC News Medical Unit.