Jan 13 (Reuters) - GSK (GSK.L) and Vir Biotechnology (VIR.O) said on Thursday they are seeking an expansion to the U.S. approval of their antibody-based COVID-19 treatment to include an option for the therapy to be given as a shot in the arm. The drug, sotrovimab, was authorised for emergency use in the United States in May to prevent mild or moderate cases of COVID-19 from worsening, and is currently being given via an intravenous infusion. Sotrovimab belongs to a class of medicines called monoclonal antibodies, which are lab-generated compounds that mimic the body"s natural defences. Tests have indicated the drug works against the fast-spreading Omicron variant. read more The new method of administration, known as intramuscular injection, was shown in a trial to work as well as the standard infusion. It offers more convenience since it can be carried out by family doctors and spare patients a trip to the hospital. read more GSK and Vir said on Tuesday the United States had agreed to buy 600,000 more doses of the therapy for an undisclosed sum. read more Products similar to sotrovimab are being offered or developed by Eli Lilly (LLY.N), Regeneron (REGN.O) and AstraZeneca (AZN.L), and so far their administration ranges from infusion, intramuscular shot to an injection under the skin. read more (This story corrected day in paragraph five to Tuesday)