It's a question that has been on the minds of researchers and oncologists long before the Pfizer and Moderna vaccine rollouts began. Although there is a consensus that the vaccines are safe for most cancer patients, according to the American Cancer Society and others in the medical community, research into whether they will be effective for cancer patients is still a data-free zone. The American Cancer Society recommends that cancer patients talk to their doctors before getting any type of vaccine because all patients and their courses of treatment are different.

There are several factors that may require a cancer patient to delay vaccination, including recent stem cell transplants or other recent use of therapy agents known to reduce vaccine efficacy, according to Dr. Laura Makaroff, the American Cancer Society's Senior Vice President for Prevention and Early Detection.

"As far as safety of the vaccine, every situation for every patient with cancer is a little different. And there's a spectrum of where any one patient might be in their cancer journey," Makaroff told CNN. "The Covid-19 vaccine is definitely safe for people with cancer but it's important that patients have a conversation with their healthcare provider and their cancer care team to determine when is the right time to have the vaccine."

"All the guidance that we're seeing -- the American Cancer society and other leading oncology groups -- is that Covid-19 immunization is recommended for patients in active therapy, but we really understand that there are limited safety and efficacy data on these patients," Makaroff said.

"Despite the lack of safety data specifically in patients with CLL (chronic lymphocytic leukemia), SARS-Cov-2 vaccination is anticipated to be safe."

CLL is characterized by a weakened immune system. The immunocompromization is so severe that CLL patients are advised to avoid live vaccines such as the ones for measles or yellow fever.

"The thing that people don't quite realize is that the impaired immune system in CLL patients -- due to the disease or some of the treatments for the disease -- can actually impact your response to vaccination," Ujjani told CNN.

"We recommend the Covid-19 vaccines for our patients ... but we're not really sure how effective it's going to be," she added. "Patients with blood cancers are typically excluded from the clinical trials evaluating the efficacy of the vaccine."

To remedy the lack of data, Ujjani is launching a research study involving 500 CLL patients across the country -- all of whom are already slated to receive the vaccine from their doctor or pharmacist -- to determine what kind of immune response they will have to the commercially available vaccines.

Advocacy groups for other more prevalent types of cancer are also encouraging cancer patients to get vaccinated as soon as possible. The Lung Cancer Action Network recently asked the US Advisory Committee on Immunization Practices to give lung cancer patients expedited access to the vaccines.

The scientific community agrees that more research is needed to determine the effectiveness of Covid-19 vaccines in cancer patients -- and many doctors are working around the clock to get more trials off the ground.

"We need these data so that we can better inform patients and better prioritize the allocation of the vaccines. If these vulnerable patients form an adequate immune response, we should certainly vaccinate them as early as possible," Dr. Elad Sharon, senior investigator at the National Cancer Institute, told CNN via email.

"But, if research efforts show that these patients fail to form a protective response to these vaccines, then what we will need to do will be to vaccinate everyone around these patients first, so that our medically vulnerable patients are most protected by the people living with them and caring for them."

Ujjani added, "We're all working really hard to answer this question, but it's hard because we've been working kind of against the clock as the vaccines just became available."

"A lot of our patients have suffered in isolation and fear, and they're not sure they're going to get back to a normal life," Ujjani told CNN. "So every oncologist is interested to see how their patients will respond to the vaccine."

Correction: An earlier version of this story misstated the academic journal that published a study conducted by the European Research Initiative. It was published in the journal Leukemia. It also misnamed the CLL Society by including the words of the acronym.

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Are Covid-19 vaccines safe for cancer patients? Here is what cancer experts say - CNN