Sales of GSKs top-selling vaccines fell sharply during the third quarter amid lower demand, sending shares in the British pharmaceutical company down 3% in U.S. trading Wednesday.GSK, which had downgraded its vaccine sales forecasts in July, did so again Wednesday in reporting earnings for the third quarter. The company now expects vaccine sales to decline by low single digits versus last year.During the third quarter, sales of GSKs shingles vaccine dropped by 7% compared to the same period in 2023, while sales of its shot for respiratory syncytial virus, Arexvy, plunged by 72% year over year.In a presentation, GSK attributed the drop of Arexvy to several factors, including a narrower recommendation from the Centers for Disease Control and Prevention, prioritization of COVID vaccinations in the U.S. and lower rates of RSV infections this season.The decline mirrored that of Pfizers competing vaccine Abrysvo, sales of which came in lower this quarter compared to last years. Pfizer also cited the CDC recommendation, which focuses on vaccination for adults 75 years or older, as well as those between 60 and 74 years who are at higher risk of disease. Previously, the CDC had urged all adults 60 and older for whom both Arexvy and Abrysvo are approved to discuss vaccination with their doctor.However, GSK also pointed to how, in the third quarter last year, there was significant stocking of Arexvy during the shots first RSV season on the market. Arexvy and Abrysvo were approved by the FDA within weeks of each other last May, but Arexvy was able to grab a significant market share advantage.Competition appears to be stiffening, however. Pfizer said Tuesday that its shots market share climbed, noting its availability as a maternal vaccination and, most recently, in certain adults 18 years or older who are at high risk.During a conference call Wednesday, GSK commercial chief Luke Miels said Arexvy maintains strong leadership market share.We're still in the foothills of this vaccine's availability and usage, said Miels. In the U.S., we continue to make data available to [the CDCs advising committee] ACIP and to support them as they move towards making longer-term recommendations on the use of RSV vaccines.Both Pfizer and GSK believe that, at some point, revaccination will be needed to maintain adequate protection in older adults.Miels added that GSK expects annual peak sales of Arexvy can eventually reach higher than 3 billion pounds.Arexvy sales during the quarter totaled 188 million pounds, or about $244 million, versus 709 million pounds last year. Sales of its shingles shot Shingrix fell to 739 million pounds, or about $959 million, from 825 million pounds last year. '