Moderna's 177% jump hits short sellers with $5.5 billion loss

Moderna shares surged 177% on Aug. 19, dealing short sellers about $5.5 billion in paper losses after the biotech reported strong late-stage trial results for a personalized melanoma vaccine developed with Merck & Co. Data from S3 Partners showed the move pushed mark-to-market losses for bearish investors to roughly $7.7 billion this year. The stock had already climbed 114% in 2026 before the trial update, as investors bet Moderna could broaden its business beyond declining Covid shot sales with newer products, including its flu vaccine. Short interest had fallen from about 20% of float earlier this year to around 14%, but the size of the rally could force more investors to cut bearish positions.