A record surge in Moderna Inc.'s shares delivered a $5.5 billion blow to short sellers who had bet that the vaccine maker's years-long slump would continue. The stock's 177% rally marked one of the most punishing squeezes in recent biotech history, catching bearish investors off guard as positive clinical data and pipeline optimism reignited buyer interest.

Short positions had ballooned during Moderna's prolonged decline, with many funds convinced the post-pandemic revenue cliff would deepen. Instead, a string of encouraging trial results for respiratory syncytial virus and influenza candidates, combined with cost-cutting measures, triggered a violent repricing. The speed of the move amplified losses as shorts scrambled to cover, creating a feedback loop that drove shares even higher.

Analysts note the squeeze reflects broader sentiment shifts toward mRNA platforms beyond COVID-19. While valuation debates persist, the episode underscores the risks of betting against clinical catalysts in volatile biotech names. Short interest has since retreated sharply, suggesting the worst of the pain may be over for remaining bears.