In a study of 1,057 patients, sotrovimab resulted in a 79% reduction in risk of hospitalization for more than 24 hours or death due to any cause, the companies said on Monday.
Similar therapies developed by rivals Regeneron and Eli Lilly have been approved by US regulators for treating non-hospitalized COVID-19 patients.
The London-listed company said it sold roughly 32 million shares in the Nasdaq-listed Innoviva for $12.25 per share, a marginal discount to the stock's closing price of $12.29 on Wednesday.
Shares of GSK were 0.3% lower at 1,361.3 pence by 1427 GMT, while Innoviva gained 5.1% to about $13.
Sanofi said it would help supply up to 200 million doses of Moderna's vaccine at its Ridgefield facility in New Jersey. Financial details of the arrangement were not disclosed.
A month later, it said it would fill and finish vials of Johnson & Johnson's single-shot vaccine at a rate of approximately 12 million doses per month.
A trial started in May last year has shown that the drug known as otilimab helps patients over 70 with severe COVID-19 get off mechanical ventilation or high-flow oxygen support faster.
The benefit for younger trial participants was not clear enough to merit further investigation, prompting the re-focus on the elderly in a follow-up trial with a targeted 350 participants.
The companies will research monoclonal antibody treatments for influenza and other respiratory illnesses as a part of the agreement.
As a part of the deal, GSK will make a further $120 million equity investment in Vir and will pay the company an additional upfront payment of $225 million.
GSK shares were down 1% at 1,347.2 pence after the drugmaker's shingles vaccine Shingrix, the biggest driver of sales growth last year, saw quarterly revenue fall 30% from a year earlier.
What we saw through the quarter ... were definitely lower vaccination rates in July and August.