GSK, in close race with Pfizer, slams brakes on phase 3 maternal RSV vaccine program over safety signal

GlaxoSmithKline has slammed the brakes on its pivotal maternal respiratory syncytial virus (RSV) vaccine program after a safety assessment, potentially handing an advantage to Pfizer in a close race to market.

RSV is an important indication for GSK, with investment in the space contributing to a recent 34% jump in spending on vaccine R&D, and 2022 is shaping up to be a key year for its attempt to claim a significant slice of a market targeted by rivals including AstraZeneca, Sanofi and Pfizer. GSK went into 2022 aiming to post data in older adults in the first half and follow up with a maternal readout by the end of the year. 

One of those targets is now in doubt. While the study in older adults is continuing, GSK has voluntarily stopped enrollment and vaccination in three maternal vaccine trials, including the 20,000-subject GRACE trial that is the centerpiece of the program.

GSK said it hit pause “following a recommendation from the Independent Data Monitoring Committee based on an observation from a routine safety assessment.” Further details on what led GSK to stop the study, how long it is likely to take to resume dosing and whether the safety signal will make the vaccine less attractive to physicians and pregnant women are yet to emerge.

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The close-run nature of the RSV race means any delay or hit to the risk-benefit profile could affect GSK’s prospects in the indication. Pfizer is the most direct challenger to GSK. Like GSK, Pfizer is running a phase 3 trial of a maternal RSV vaccine that is close to delivering data. Pfizer lists both the adult and maternal trials of its RSV vaccine among the pivotal studies that are scheduled to read out in the first half of 2022.

By vaccinating pregnant women, GSK and Pfizer hope to protect their infants from lower respiratory tract infections over the first six months of their lives. AstraZeneca and Sanofi are tackling the problem from a different angle, developing a long-acting antibody for administration to infants. The partners have shown the passive immunization protects infants, setting them up to file for approval this year.  

Depending on the duration of the delay and severity of the signal, GSK could come to market well after its rivals with an asset that has real or perceived safety concerns. Equally, GSK could bounce back quickly and emerge as a force in the RSV space.