Jan 21 (Reuters) - GlaxoSmithKline (GSK.L) and U.S. partner Vir Biotechnology (VIR.O) will boost production of their antibody-based COVID-19 treatment by adding a second manufacturing plant to help meet soaring demand in the United States. The U.S. government last week signed a deal with the drugmakers to buy 600,000 more doses of the therapy, sotrovimab, for an undisclosed sum. read more The GSK-Vir therapy has shown more promise against the Omicron variant in lab studies than treatments from major rivals Eli Lilly (LLY.N) and Regeneron (REGN.O). read more "GSK had been planning to introduce another production facility to our manufacturing network in advance of Omicron," a GSK spokesperson told Reuters on Friday. "The second site provides GSK additional scale and diversity given the pandemic uncertainty." Deliveries of the 600,000 additional doses will be carried out through the first quarter of 2022, the spokesperson said. The Wall Street Journal reported on Friday that GSK and Vir were expected to make the first deliveries in February. The United States is reporting an average of 752,698 new coronavirus infections a day, according to a Reuters analysis of official data, fuelling the urgent need for effective solutions. The Biden administration in November signed contracts worth about $1 billion for an unknown number of doses of sotrovimab before the rapid spread of Omicron, and has said it would control the distribution of the drug in the country.