The conclusion stems from the experience of 54 cancer patients who developed COVID-19.
Sixty-five percent were hospitalized following infection, while nearly 1 in 5 (19%) were placed on a mechanical ventilator for breathing assistance. In all, 13% died.
"We hoped that the vaccines would be highly protective â€” at least that was our theory," said study author Dr. Jeremy Warner. "So yes, we were surprised and disappointed to see that patients were still getting pretty sick and, sometimes, dying."
Still, the findings reflect a seven-month stretch beginning in November of last year.
That, the study team noted, was before the U.S. Centers for Disease Control and Prevention recommended that all cancer patients get booster shots. It was also before the emergence of the new and not yet fully understood Omicron variant.
Given the fast-changing COVID-19 landscape, both factors may have a significant impact on how fully vaccinated cancer patients will fare going forward.
"Getting a booster is more important than ever," said Warner, an associate professor of medicine in hematology/oncology at Vanderbilt-Ingram Cancer Center in Nashville, Tenn.
He noted that some countries â€” such as Israel -- "no longer consider un-boosted people to be fully vaccinated."
"As for Omicron, evidence now seems to indicate that it is less severe for general populations," Warner added. "But we donâ€™t know if that will translate to patients with cancer, especially those that we have seen at highest risk," meaning those who are older, male, have multiple health issues, and/or fast-spreading cancer.
There is hope that recently approved antiviral pills, one from Pfizer Inc. and one from Merck & Co., might offer more protection to the most vulnerable: Last week, the FDA approved Pfizer Inc.'s Paxlovid for emergency use in high-risk people after trial results showed it slashed the risk of hospitalization and death by nearly 90% in such patients.
But even with the question of boosters, Omicron and antiviral pills taken out of the equation, Warner said he and his colleagues were dismayed by the degree of serious COVID they saw among cancer patients.
The study involved 129 U.S. research centers that joined forces as the COVID-19 and Cancer Consortium â€” or CCC19, for short.
For this study, the CCC19 group honed in nearly 1,800 cancer patients who developed COVID-19 between Nov. 1, 2020, and May 31, 2021.
The vast majority had not been vaccinated.
But 54 patients had been fully vaccinated, meaning twice with the Pfizer or the Moderna vaccine, or once with the single-dose J&J vaccine.
Even so, nearly two-thirds ended up sick enough with COVID that they needed to be hospitalized.
The researchers offered one possible explanation: that nearly half (46%) of the 54 vaccinated patients had reduced levels of key immune system cells (lymphocytes) that are critical to the body's ability to fight off infections.
Cancer treatment often triggers a drop in such cells, Warner and his associates said. As a result, such patients simply can't mount the same immune response that COVID vaccines have been found to trigger among those who are not battling cancer, the team said.
So what can such vulnerable patients do to lower their risk?
"A simple precaution is to upgrade masks," Warner advised. "There are now pretty comfortable KN95 and KF94 options. They might not be quite as protective as N95, but are much better than cloth and likely surgical masks."
And given that many cancer patients are struggling with a weakened immune system, "it remains very important for all [of their] close contacts to be vaccinated and boosted," he added.
Lastly, frequent testing is an important part of the defense for patients, as well as their friends and family â€” especially when they're planning to spend time together, Warner said.
Researchers stressed that while their findings are concerning, the small number of patients studied make it impossible to draw firm conclusions as to why so many of the vaccinated patients became seriously ill with COVID.
That thought was echoed by Dr. Julie Gralow, chief medical officer with the American Society of Clinical Oncology in Alexandria, Va., who characterized the findings as "worrisome."
"We must â€¦ continue to focus on doing everything we can to prevent patients with cancer from exposure to COVID and the particularly contagious Omicron variant," she said.
She noted that many of the vaccinated patients in the study appeared to have relatively aggressive cancer and had undergone relatively aggressive treatment. So it's possible, Gralow said, that the high risk for serious breakthrough infection and death might have something to do with the relatively serious nature of their cancer.
"We will need to keep an eye on this trend, and see if it holds up with longer follow-up/more recent data, and larger numbers of vaccinated patients," she said.
The findings were published Dec. 24 in the Annals of Oncology.
The U.S. Centers for Disease Control and Prevention has more about breakthrough infections.
Annals of Oncolog