Moderna's stock slumps after RSV shot efficacy wanes faster than GSK's Arexvy

The efficacy of Moderna’s investigational respiratory syncytial virus (RSV) vaccine appears to wane faster than GSK’s approved shot Arexvy, a revelation that sent the Big Biotech’s stock slipping.

Phase 3 data shared (PDF) Thursday showed overall efficacy of 63.3% after 8.6 months in preventing RSV-associated respiratory tract disease with two or more symptoms, dropping from 84% at 3.3 months. The three-month data were used to gain breakthrough status for the investigational vaccine, dubbed MRNA-1345, more than a year ago.

The biotech’s stock, which was $96.60 per share Thursday at market open, has fallen significantly since the data were posted, settling at $87.70 as of 2 p.m. ET Friday.

The new analysis included data from more than 36,000 participants, with median vaccine efficacy rates of 63.3% for RSV illnesses with two or more symptoms; 63% for three or more symptoms; and 53.9% against RSV-associated acute respiratory disease at an 8.6-month follow-up.

The follow-up data compare to GSK’s Arexvy, which is approved for patients above 60 years and showed 82.6% overall efficacy that fell to 77% in a second season, equating to a cumulative 67.2% efficacy over two seasons.

Meanwhile, Pfizer’s Abrysvo has shown 66.7% efficacy in the first year against RSV. In the second season, at Month 18, the efficacy numbers wane to 49%.

Moderna discouraged comparing the shots, noting that different RSV seasons make it difficult to pull direct comparisons, according to a Feb. 8 statement on the RSV data. The endpoints and other details across the studies were also not a perfect match. 

“In the absence of head-to-head clinical trials, comparative conclusions regarding the safety and efficacy of mRNA-1345 relative to other RSV vaccines cannot be made,” Moderna said. “These trials differed in study populations, geographic locations, infection surveillance methods, and case definitions used for RSV.”

The mRNA company still expects the vaccine to receive regulatory approval in the first half of this year.