GSK PLC (LSE:GSK, NYSE:GSK) shares plunged close to 7% following a ruling in the US that is likely to restrict the use of its respiratory syncytial virus (RSV) vaccine.
The US Centers for Disease Control and Prevention's (CDC) advisory committee on immunization practices (ACIP) voted to restrict all those over the age of 75 to a single dose of the treatment.
Those aged 60 to 74 and are at severe risk of RSV should also only get a single dose, limiting GSK's market for the vaccine compared to the prior blanket 60-plus recommendation.
RSV can cause infections to the lungs and the respiratory tract and is common in infants and older adults.
The committee saw a potentially elevated risk of neuroinflammatory condition Guillain-Barré syndrome (GBS) as their reasoning.
This was despite evidence suggesting Arexvy does not statistically increase risk, analysts noted, suggesting the GSK sell off might be overdone.
Moderna, which presented RSV data to the ACIP that analysts said was less competitive at 50% protection compared to GSK's Arexvy's 67%-75%.
Moderna shares plunged 11% in New York.