Roche’s multiple sclerosis newcomer Ocrevus was approved by the FDA last March bearing a warning about immunization. Now, researchers have gathered clinical data showing it could weaken the effectiveness of certain vaccines.

In a Roche-sponsored phase 3b study, scientists examined over 100 MS patients’ responses to vaccines against tetanus, seasonal flu, pneumococcus and a neoantigen called keyhole limpet hemocyanin, and compared the data between those on Ocrevus and those not on the drug. As it turned out, patients who received Ocrevus showed lower levels of immune responses to the vaccines across the board.

To be specific, the tetanus shot induced antibody responses in about 24% of those treated with Ocrevus, as compared to 55% in those who weren’t on the drug. Seroprotective titers against five influenza strains ranged from 55.6% to 80% in the Ocrevus group, compared with 75% to 97% for those not on the drug.

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“This study shows that while people with MS treated with ocrelizumab can still mount vaccine responses, it's not nearly as strong as prior to treatment,” Penn Medicine neurologist Amit Bar-Or, M.D., said in a statement. Bar-Or presented the findings at the 2018 American Academy of Neurology annual meeting in April.

These results didn’t come as a surprise, mainly because of the mechanism of action for Ocrevus. A monoclonal antibody, Ocrevus lowers the activity of CD20-expressing B cells to reduce MS attacks. Because B cells produce antibodies key in humoral response, reducing their count could mean a weakened immune system that's less responsive to vaccines.

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In Ocrevus’ label, patients are asked to get vaccines at least six weeks before they start treatment. And it is recommended they do not get live or live-attenuated vaccines while on the drug or before their B-cell levels return to normal. How long will that be? In a previous study of 51 patients, the median time for B-cell recovery was a year and a half.

Bar-Or noted that Ocrevus-treated patients’ responses to vaccines, though lower, are still important, especially against seasonal infections such as the flu. “[I]t appears safe for patients taking ocrelizumab to get vaccinated and vaccination is likely to provide them with at least some protection from such infections,” he said.

Industry watchers have high hopes for Ocrevus because it’s the first product ever to win approval for hard-to-treat primary progressive MS. For the first quarter, sales of the drug reached CHF479 million ($486 million), making it one of the best-performing MS launches in industry history.