Proactive Investors - GSK PLC (LON:GSK) shares fell 6% after US authorities recommended restrictions on doses of any respiratory syncytial virus (RSV) vaccine.
Pfizer (NYSE:PFE) and Moderna (NASDAQ:MRNA), which also make RSV vaccines, fell 2% and 11% in New York overnight.
The US Centers for Disease Control and Prevention's (CDC) advisory committee on immunization practices (ACIP) unanimously voted to restrict RSV vaccine recommendation to defined at-risk 60-74s, whilst expanding use for all 75 and older.
These decisions may cut the 2024-25 addressable US market from around 80 million people aged 60-plus as well as circa 13 million at-risk aged 50-59, to perhaps circa 55 million, said analysts at Jefferies, with around 10 million dosed last season.
ACIP voted unanimously in favour of all adults aged 75 and older to receive a single dose of RSV vaccine and that adults aged 60-74 who are at-risk of severe RSV receive only a single vaccine, replacing the prior blanket 60-plus recommendation under shared clinical decision-making.
The committee saw a potential elevated risk of neuroinflammatory condition Guillain-Barré syndrome (GBS) as their reasoning.
This was despite evidence suggesting Arexvy does not statistically increase risk, the Jefferies analysts noted, adding that as additional real-world safety data accumulate "the recommendation may be expanded".
A vote on Arexvy for at-risk 50-59s was postponed, as the ACIP awaits further data on GBS risks, affecting a small part of forecast peak US doses.
Moderna RSV data was presented to the ACIP, which Jefferies said was "as expected", though the data was less competitive at 50% protection compared to GSK's Arexvy's 67%-75%.