China's largest vaccines company Zhifei will pay British drugmaker GSK 2.5 billion pounds ($3.05 billion pound) for the exclusive rights to distribute GSK's shingles vaccine in the world's No.2 pharmaceuticals market.

The vaccine, named Shingrix, is GSK's top-selling medicine. The deal with Zhifei is part of its efforts to reach more 4 billion pounds ($4.88 billion) in sales by 2026, GSK said.

Zhifei will purchase volumes of Shingrix with a value to GSK worth 2.5 billion pounds over an initial three-year period, it said in a filing to the Shenzen stock exchange.

The agreement is set to start on January 1. Zhifei shares were up as much as 20% to 58.40 yuan following the filing, their highest point since March 27.

GSK's statement also said that Zhifei has granted it the "right of first refusal to be their exclusive partner for any co-development of an RSV vaccine for older adults in China".

The company launched its respiratory syncytial virus (RSV)vaccine, called Arexvy, in the United States after it was approved by regulators in May.

Barclays analysts said in a note last month that the first five weeks of prescription data showed that the vaccine's launch was off to a strong start and that GSK appeared to have the lead over Pfizer's competing vaccine for RSV.

A GSK shareholder then told Reuters that if the strong start was maintained through September, it would mean consensus estimates for 2023 sales of Arexvy, at about 215 million pounds, are "way too low".

GSK is banking on Arexvy being its next blockbuster medicine, part of its efforts to ensure growth as it faces a combination of patent expiries and declining revenue from its current bestsellers by the end of this decade.

($1 = 0.8192 pounds) (Reporting by Maggie Fick in London and Andrew Silver in Shanghai, additional reporting by Eva Mathews in Bengaluru; Editing by Savio D'Souza)