Moderna stock crashes as leaked melanoma data threatens its record August rally

Two months ago, Moderna's ( MRNA ) melanoma vaccine handed the stock its best day ever. Now leaked results from the same trial have handed it its biggest intraday drop on record.

The stock fell as much as 26% before closing down more than 17%. That's still its worst day since Aug. 20, the session right after its best day.

More than $17 billion in market value has vanished as details from the late-stage trial circulated ahead of their official debut . Merck ( MRK ), Moderna's partner on the vaccine, confirmed the results came out early. The full data is set for release Saturday at Europe's biggest cancer conference, in Madrid.

The leaked numbers are still good. The personalized vaccine, paired with Merck's blockbuster Keytruda, reportedly cut the risk of the cancer returning or death by 42%. An earlier, smaller trial showed 49%, though it used different patients, so it isn't a direct comparison. The bigger issue is how much the stock had already run, nearly quadrupling from mid-August to Wednesday.

Merck fell nearly 4% at its low before paring more than half the drop. The vaccine is central to Moderna's story, but it's just an add-on for Merck.

The rest of healthcare is rallied. Eli Lilly ( LLY ) is added more than $33 billion in market value, about twice what Moderna lost. Johnson & Johnson ( JNJ ) and Pfizer ( PFE ) rose, too.

The iShares Biotechnology ETF ( IBB ) and SPDR S&P Biotech ETF ( XBI ) were only modestly lower, roughly in step with the S&P 500 ( ^GSPC ).

On Aug. 19, Moderna rocketed 177% to close at $174.38 after the company said the trial met its goals. It kept climbing from there, and on Wednesday, it posted its highest close since December 2021.

Thursday's low came within 4% of that August close before buyers stepped in. A sustained break below $174 would put $116 in focus. That's where trading opened on the record day.

Watch Saturday's full data, especially how lower-risk patients fared.

Jared Blikre is the global markets and data editor for Yahoo Finance. Follow him on X at @SPYJared or email him at jaredblikre@yahooinc.com .