Feb 5 (Reuters) - GSK posted a better-than-expected fourth quarter on Wednesday and lifted its 2031 sales target to nearly $50 billion as the British drugmaker aims to invest heavily in its specialty medicines to offset weakness in vaccine.

GSK's fourth-quarter earnings topped estimates as strength in its HIV and oncology portfolio offset a fall in sales of its respiratory syncytial virus (RSV) and shingles shots.

"We are increasing and prioritising R&D investment to promising new long-acting and specialty medicines in Respiratory, Immunology & Inflammation, Oncology and HIV," CEO Emma Walmsley said in a statement.

Walmsley's bet on blockbuster RSV vaccine, Arexvy, has hit hurdles in the United States, including changes in public health agency recommendations, prioritisation of other vaccinations such as COVID-19 and lower seasonal infections.

Since taking the helm of GSK in 2017, Walmsley has returned the company to growth with a focus on cancer and infectious diseases to counter a combination of patent expiries and declining revenue from its bestsellers by 2030.

GSK expects revenue to grow between 3%-5% in 2025, a slower pace compared with the 7% growth it reported on Wednesday for 2024, mainly due to lower sales of its vaccines in the Unites States.

Analysts had expected sales to grow about 3.5% this year, according to a company-compiled consensus.

The British drugmaker lifted its 2031 sales forecast to more than 40 billion pounds ($49.88 billion), from 38 billion pounds it targeted earlier.

The company reported core earnings per share of 23.2 pence on sales of 8.12 billion pounds for the quarter ended Dec. 31, compared with 19 pence on sales of 7.75 billion pounds expected by analysts in an LSEG poll.

GSK also said it would buy back 2 billion pounds worth of shares over the next 18 months.

Revenue from its vaccine business in 2025 is expected to see a decrease of a "low single-digit per cent", it said.

($1 = 0.8017 pounds)