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.