Key Highlights

● Moderna was trading at $202.63, the worst performer in the S&P 500 on Thursday.

● About $24 billion of Moderna's market value was erased as Phase 3 data emerged early.

● The vaccine combination showed a hazard ratio of 0.58 in more than 1,100 melanoma patients.

● Health care still climbed to a record high as the S&P 500 fell 93.74 points.

The big scientific moment for Moderna (NASDAQ:MRNA) was supposed to come on Saturday. Instead, Wall Street got an early look on Thursday, and the reaction was brutal.

Shares of Moderna plunged on Thursday, October 22, with the stock trading at $202.63, the worst performance in the S&P 500. At one point the stock was down even more steeply, trading near $183, which according to market data would have been its steepest one-day drop on record. About $24 billion in market value was erased.

The trigger was Phase 3 data for its experimental melanoma vaccine, developed with Merck (NYSE:MRK), which appeared before its scheduled presentation at the European Society for Medical Oncology congress in Madrid. The data was reportedly leaked.

What the data showed

The INTerpath-001 trial tested intismeran autogene, a personalized cancer vaccine, in combination with Merck's immunotherapy Keytruda in more than 1,100 patients with high-risk melanoma.

The combination reduced the risk of recurrence or death, with a hazard ratio of 0.58, and also reduced the risk of distant metastasis or death. But the benefit was smaller than in the earlier Phase 2b study, which had set expectations high.

Sell the news

Analysts largely said the data was good, while the stock reaction reflected lofty expectations. One analyst called it "solid data" and described the move as a "sell the news" event. Another also called the data great and described the reaction in similar terms, and a third firm called the results compelling.

The stock had risen more than sixfold this year before Thursday. Merck also slipped, trading at $147.24, as investors treated the readout as a sell-the-news event for the partner.

Why ESMO matters

ESMO is one of the world's leading cancer conferences, and full presentations there typically include far more detail than headline figures. On Saturday, investors may get more detail, such as how the vaccine performed across patient subgroups, the safety profile and the durability of responses over time.

Those details could shape how doctors and regulators view the therapy. A personalized cancer vaccine is a new approach that tailors treatment to each patient's tumor, and its commercial prospects depend on more than one headline number.

A sector that held up

Moderna's slide did not derail the broader health care sector, which climbed to a record high on Thursday. Johnson & Johnson and Amgen rose as investors rotated into defensive stocks. Quest Diagnostics beat estimates and raised its full-year earnings guidance to $11.25 to $11.35 a share.

Appetite for new biotech issues also remained. Lycia Therapeutics priced an upsized initial public offering at $18.00 a share, raising about $317.5 million, ahead of its Friday debut.

The broader market was split. The S&P 500 fell 93.74 points to 7,663.43, its worst day since July, as AI and chip stocks sold off. The Dow Jones Industrial Average edged up 11.89 points to 52,867.98, and about two of every three S&P 500 stocks rose, according to market data.

Saturday's Subgroup Test

The full presentation at ESMO in Madrid on Saturday is the key event. Investors will focus on subgroup results, side effects and how long the benefit lasts. Monday will bring the first trading session after the release, when Moderna and Merck shares will reflect the full data. Any comments from the companies on regulatory plans for the vaccine will also be closely followed.