Researchers analyzed the vaccine’s effectiveness at 18 months in the 16 people who received it, eight of whom responded and eight of whom didn’t. The non-responders went a median of 13.4 months between their surgery and their cancer’s recurrence, while not enough of the responders had had their disease return to calculate a median figure. Researchers determined the risk of recurrence in the responders was 92% lower than among the non-responders.

Importantly, they found that patients’ T cell response persisted, lasting up to two years despite the follow-up chemotherapy. Half of the treated participants had T cells that were specific to at least one neoantigen. (The median number of neoantigens bound by T cells after vaccination was two, and the highest eight.)

Side effects were few overall, researchers said. One patient who received autogene cevumeran experienced fever and hypertension that qualified as “Grade 3,” or severe.

“Although a very small study, this trial demonstrates the ability of mRNA vaccines to elicit potent anti-tumor T cell responses that appear to create significant clinical benefit,” said Ira Mellman, vice president of cancer immunology at Roche’s Genentech division and an author on the paper, in an emailed statement. “That such benefit was seen in pancreatic cancer, a lethal disease that has thus far been refractory to any form of immunotherapy, makes the result all the more exciting.”

Mellman said the study also suggests the best use for cancer vaccines may be in the post-surgical setting, when patients and physicians are trying to keep disease from returning, rather than in primary treatment of tumors.

In an accompanying Nature editorial, Amanda Huff and Neeha Zaidi of Johns Hopkins University medical school wrote that the study “established the feasibility of using mRNA-based neoantigen vaccines for pancreatic cancer.”

Yet they cautioned that researchers need to determine why half of the people given the vaccine didn’t respond.

The paper’s authors suggest some possible explanations. Non-responders had, on average, larger primary tumors. Five of the eight also had their spleens removed as part of their pancreatic cancer surgery, compared to two of the responders. Mice that have their spleens removed have weaker T cell responses to mRNA vaccines, they noted.

“That vaccines induced high magnitude T cell responses in 50% of patients may highlight the need for biomarkers to select optimal patients and tumors for this treatment,” they wrote.