(Bloomberg) -- GSK Plc slumped after US health officials recommended restricting vaccination with its RSV shot to people who are older and more at risk in a move that could reduce the market for the UK drugmaker’s blockbuster vaccine. 

The shares fell as much as 7.2% in London trading, wiping out most of their gains for the year.

The guidance marks the second blow to GSK this week after it lost out to rival Pfizer Inc. on a contract to supply millions of doses of the vaccine to its home country. 

An advisory panel to the US Centers for Disease Control and Prevention only recommended vaccination in adults aged 60 to 74 if they were at risk of severe disease. RSV, or respiratory syncytial virus, is common but sometimes deadly, especially in the very young and the elderly.

For those aged 75 and over, the panel voted in favor of vaccination — a stronger endorsement than the prior recommendation. 

The committee previously advised that all those aged 60 and over should speak to their doctors about whether the shot would be beneficial to them. GSK’s vaccine Arexvy is approved in the US for at risk 50 to 59 year-olds, but the group postponed recommendations for this age group. 

The intention “is to increase vaccine uptake, but market concerns may be such that it has the opposite effect,” said Bloomberg Intelligence’s John Murphy. 

Pfizer and GSK have been locked in a fierce rivalry over take-up of their RSV shots. GSK has managed to corner a larger portion of the market in the US but Pfizer beat the British drugmaker to a tender that will see the shot rolled out on the UK’s health service. As the publicly broadcast CDC committee met on Wednesday, Pfizer shares fell 2%. 

--With assistance from Lisa Pham.

(Updates with detail of Pfizer’s vaccine in eighth paragraph)

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