Moderna delays norovirus vaccine trial readout because of too few infections

Biotech giant Moderna has delayed the readout of its Phase 3 trial for its mRNA-based norovirus vaccine, mRNA-1403 (NOVA 301), because the number of norovirus infections observed so far has been too low to meet statistical requirements for an early efficacy signal.

The company first launched a two-season global Phase 3 trial (Northern and Southern Hemisphere) to capture enough cases of moderate or severe norovirus gastroenteritis, but fell short on case accrual in the Southern Hemisphere, prompting the trial to return for a second Northern Hemisphere season.

At a planned interim analysis after the second norovirus season, mRNA-1403 did not cross the pre-specified statistical bar for early success, so the trial remains blinded and ongoing, with Moderna planning to enroll an additional cohort to gather more cases over at least one more winter season.

Moderna now expects a definitive Phase 3 readout no earlier than 2027–2028, as the final timeline depends on the unpredictable circulation of norovirus and the ability to accumulate sufficient infection endpoints.

The trial previously faced a separate FDA clinical hold in early 2025 due to a reported case of Guillain‑Barré syndrome, and while the hold was later lifted, the combined issues of case accrual and statistical thresholds have delayed any near-term commercial prospect for the first-ever approved norovirus vaccine.