A newly developed COVID-19 vaccine induces a robust immune response in immunocompromised patients, including those with cancers like leukaemia and lymphoma, according to early results from a small trial.

The findings, presented at the American Academy of Cancer Research (AACR) Annual Meeting in Louisiana, showed that the vaccine, called CoVac-1, induced T-cell immune responses in 93 per cent of patients with B-cell deficiencies.

"To our knowledge, CoVac-1 is currently the only peptide-based vaccine candidate specifically developed and evaluated for immunocompromised patients," said Juliane Walz, senior author of the study, and a professor at the University Hospital Tubingen in Germany.

A peptide vaccine is the one where the protein pieces are injected directly, rather than being encoded via messenger RNA (mRNA). The researchers noted that while vaccination induces a robust immune response against the SARS-CoV-2 virus in the majority of individuals, approved vaccines have shown decreased efficacy in many immunocompromised people.

Patients undergoing treatment for blood cancers represent one such population, as their treatment regimens often damage healthy immune cells, particularly B cells, in addition to malignant ones, they said.

"In the clinic, we see many cancer patients who do not mount sufficient humoral immune responses after vaccination with available SARS-CoV-2 vaccines. These patients are thus at high risk for a severe course of COVID-19," Walz said in a statement.

Many chemotherapies and some immunotherapies destroy B cells, the immune cells responsible for humoral or antibody-mediated responses. Currently approved SARS-CoV-2 vaccines rely heavily on humoral responses, which may be impaired in patients with a B-cell deficiency. One way to compensate for this is to enhance the response from T cells, another type of immune cell.

"T-cell immune responses against SARS-CoV-2 are of particular importance for patients with B-cell deficiencies, who develop very limited antibody responses after infection or vaccination," said Claudia Tandler, a graduate student at the University of Tubingen, who presented the study.

"T cell-mediated immunity is indispensable for developing protective antiviral responses, and previous evidence has shown that T cells can combat COVID-19 even in the absence of neutralising antibodies," Tandler said. Designing a vaccine to stimulate T cells, Tandler explained, requires the careful selection of SARS-CoV-2 antigens -- small pieces of viral proteins that can stimulate immune cells.

While the current mRNA-based vaccines produce a larger piece of a single protein -- the spike protein -- which our cells can break down into antigens, the researchers chose six specific antigens from different parts of the virus, and not limited to spike, to make up their vaccine.

"CoVac-1-induced T-cell immunity is far more intense and broader, as it is directed to different viral components than mRNA-based or adenoviral vector-based vaccines that are limited to the spike protein and are thus prone to loss of activity due to viral mutations," Tandler said.

In the phase I trial, the researchers recruited 14 patients with a B-cell deficiency, including 12 patients with leukaemia or lymphoma. The patients were given a single dose of CoVac-1 and monitored for up to six months for safety and immune response.

As many as 64 per cent of the participants had been previously vaccinated with an approved SARS-CoV-2 vaccine that failed to elicit a humoral immune response, the researchers said. Fourteen days after vaccination, T-cell immune responses were observed in 71 per cent of patients, which rose to 93 per cent of patients 28 days after vaccination, they said.

The researchers measured the potency of CoVac-1-induced T-cell responses and found them to exceed spike-specific T-cell responses observed in B cell-deficient patients after vaccination with mRNA vaccines. T-cell responses from CoVac-1 also exceeded those mounted by individuals who are not immunocompromised following a SARS-CoV-2 infection, they said.

The researchers are currently preparing a phase III clinical trial to evaluate CoVac-1 in a larger population of immunocompromised individuals. They are hopeful that the results will allow the vaccine to protect cancer patients with B-cell deficiencies from severe cases of COVID-19.

"CoVac-1 is designed to induce broad and long-lasting SARS-CoV-2 T-cell immunity, even in individuals who have impaired ability to mount sufficient immunity from a currently approved vaccine, and thus protect these high-risk patients from a severe course of COVID-19," Walz said.

The researchers acknowledged some limitations of their study, including a relatively small sample size with low racial and ethnic diversity.