With sales of Roche’s Tamiflu cratering after it went generic three years ago, the company is anxiously monitoring successor flu drug Xofluza’s chances. Now, the drug has scored new preventative data that could nudge sales upward.

A single dose of Xofluza beat placebo at preventing the onset of symptoms in patients exposed to a family member with the flu, according to a phase 3 trial released this week. Only 1.9% of Xofluza patients developed symptoms, compared with 13.7% of patients taking a placebo.

Roche plans to take those results to regulators for a potential new approval. “Preventing otherwise healthy people from developing the flu virus will reduce the overall societal burden of disease, and we look forward to sharing these data with health authorities around the world,” said Sandra Horning, R&D chief at Roche’s Genentech unit.

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Xofluza’s trial win could help Roche absorb the blow from Tamiflu’s continuing slide after generics hit the market in 2016. In the first quarter, Tamiflu posted a 40% sales drop from the same period the previous year, down to $180 million.

Xofluza hasn’t pitched in much since its approval late last year. In the first quarter, it posted a measly $6.06 million for Roche—all in U.S. sales. But there’s hope: Partner Shionogi, which owns the rights in Japan, cleared $243 million in domestic sales in fiscal 2018.

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Still, Xofluza is not only fighting an uphill battle against established Tamiflu generics but also must contend with some troubling Japanese data indicating the drug’s vulnerability to mutant strains of the flu.

As of February, the National Institute of Infectious Diseases in Tokyo had already identified six resistant strains, casting a shadow over the novel drug’s future chances on the global stage. Those results backed up Shionogi’s initial clinical data, which found that 25% of children in late-stage trials had resistant strains of the flu.

Roche in February said while mutant strains had been identified, they only showed “reduced susceptibility” to Xofluza instead of full resistance, meaning they were not classified as resistant by U.S. regulators.

Xofluza also stands to lose from clinical trial data showing statistically insignificant improvement of flu symptoms over time compared with Tamiflu. In a phase 3 trial, dubbed Capstone-2, Xofluza showed improvement of symptoms in patients at 73.2 hours, topping Tamiflu’s 81 hours but still not enough to hit the trial’s primary endpoints. That result mirrored earlier Capstone-1 trial data that found little difference in symptom improvement time between the drugs.