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 — 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 — 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 don’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 — 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.