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Moderna (MRNA) Soared 177% on a Cancer Vaccine…

Moderna (NASDAQ: MRNA) closed up about 177% on Wednesday, August 19, at $174.38, its largest one-day gain on record, after the company and Merck (NYSE: MRK) reported the first-ever positive Phase 3 result for an mRNA-based cancer vaccine. The move added roughly $44 billion in market value in a single session, taking Moderna from about $25 billion to $69 billion. By Thursday’s pre-market the stock had eased back toward $152, giving up part of the surge, but the scale of the reaction told the story: one clinical readout changed how the market sees a company it had spent two years writing off.

Beyond the share price, the result is the clearest evidence yet that mRNA, the technology behind Moderna’s Covid vaccine, can work against cancer, and it arrived for a company whose Covid revenue had collapsed and whose stock had lost most of its value from the pandemic peak. Whether the rally holds is a separate question from whether the science is real, and both deserve a clear look.

Moderna’s near-vertical move on August 19 from about $63 to a $174 close, with the stock easing in Thursday’s pre-market. Source: TradingView

What Moderna and Merck Announced

The two companies said their Phase 3 INTerpath-001 trial met its primary endpoint. The study tested intismeran autogene, an individualized mRNA cancer vaccine, in combination with Merck’s immunotherapy Keytruda, in 1,137 patients with completely resected stage IIB-IV melanoma, randomized two-to-one to the combination or Keytruda alone. At a pre-specified interim analysis, patients who received the combination went longer without their cancer returning than those on Keytruda alone, meeting the primary goal of recurrence-free survival, and the trial also hit a key secondary endpoint of distant metastasis-free survival. The study is continuing to evaluate overall survival.

The vaccine is built from a sample of each patient’s own tumor and designed to teach the immune system to recognize up to 34 mutations specific to that person’s cancer. What makes the readout historic is the category: according to Merck and Moderna, it is the first positive Phase 3 result for an individualized neoantigen therapy and the first for any mRNA-based cancer therapy. Moderna co-founder Noubar Afeyan framed the milestone directly.

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CEO Stéphane Bancel called the findings “a pivotal moment for the field of cancer research,” noting that the idea of an mRNA treatment designed for one patient’s tumor was “aspirational” not long ago. The results build on the earlier KEYNOTE-942 Phase 2 study, whose five-year data showed a 49% reduction in the risk of recurrence or death and a 59% reduction in the risk of distant metastasis or death versus Keytruda alone. Melanoma is a serious target: about 112,000 new US cases and more than 8,500 deaths are projected for 2026, according to the companies’ announcement.

The Biggest One-Day Move in the MRNA Stock’s History

The market’s response was violent. Moderna closed up about 177%, from a Tuesday close of $62.96 to $174.38, on volume of 185.1 million shares, roughly 1,819% above its three-month average. Merck, far larger at around $333 billion, rose about 12.6%, and the rest of the mRNA field rallied in sympathy: BioNTech (NASDAQ: BNTX) climbed about 22%, Novavax (NASDAQ: NVAX) about 4%, and Pfizer (NYSE: PFE) about 4%.

In addition to melanoma, the companies said the broader INTerpath program now spans nine Phase 2 and Phase 3 trials across multiple tumor types, including non-small cell lung cancer, bladder cancer, and renal cell carcinoma, and the market was pricing in the possibility that a platform validated in one setting could work in others. That optionality, rather than melanoma alone, is what a move of this size reflects

Investor Takeaway

The 177% figure is Wednesday’s closing gain from a low base of $62.96, not the current level, and the stock was already easing toward $152 in Thursday’s pre-market, so the move should be read as a record one-day reaction rather than a new floor.

The $5 Billion Short Squeeze

Few stocks were as heavily bet against as Moderna, which is why the surge was so costly for bears. Short sellers took roughly $5 billion in mark-to-market losses in a single day, per Kalshi, with Bloomberg citing about $5.5 billion from S3 Partners and ORTEX data pointing to around $4.8 billion, a record one-day loss for the stock. The bet against Moderna had a clear logic: with Covid revenue gone, skeptics wagered the company could not replace it. The cancer readout offered the first real answer to that thesis.

The squeeze was amplified because the trade was already unwinding. Short interest had fallen from around 20% of Moderna’s float earlier in 2026 to roughly 14%, with about a quarter of bearish positions covered during the year. A surge of this magnitude forces remaining shorts to reassess, adding buying pressure on top of the fundamental move.

A Biotech Crashes the 2026 Leaderboard

The rally vaulted Moderna to the second-best-performing stock in the S&P 500 this year, up more than 400% year to date, trailing only SanDisk. What stands out is the company it keeps: the 2026 leaderboard is otherwise dominated by the memory and semiconductor names riding the AI infrastructure trade.

Moderna is the only biotech near the top of a 2026 leaderboard otherwise built on the AI and memory trade. Source: slickcharts.com · Chart: FinanceFeeds

Memory and storage names lead it after a historic upcycle, with SanDisk (SNDK) at the top and Micron (MU) and Marvell (MRVL) close behind on AI-driven demand. Moderna is the exception, a biotech surging on science rather than silicon, and its arrival near the top is a reminder that 2026’s biggest single-stock moves have not all come from the AI trade. The breakthrough also drew reaction well beyond biotech, including from Elon Musk, replying to scientist Eric Topol.

What It Does and Doesn’t Prove

The science is a genuine milestone, but several things temper the stock reaction. The data released Wednesday came from a pre-specified interim analysis, with the trial still evaluating overall survival; the full results will be presented at an upcoming medical meeting, and the companies still need to file for regulatory approval. Notably, the companies did not release the magnitude of the Phase 3 benefit, no hazard ratio, no p-value, only the statement that the improvements were statistically significant and clinically meaningful, so the size of the effect is not yet public.

Moderna and Merck share the program’s economics under a 50-50 profit split, so the revenue does not flow to Moderna alone. Wall Street’s own response was more measured than the tape: analysts issued upgrades but most set price targets below Wednesday’s close, with BofA moving to Neutral at a $170 target while Goldman and Morgan Stanley stayed neutral on valuation grounds. FinanceFeeds lays out the full range in its Moderna bull-versus-bear breakdown, a $170 bull case against a $60 bear case.

The move even reached disclosed political portfolios, with filings compiled by Quiver Quantitative and reported by Benzinga showing President Trump bought Moderna stock in the $15,000 to $50,000 range on March 2, 2026. The position gained roughly $27,000 to $91,000 on Wednesday, while the White House says his holdings sit in a blind trust. It is a measure of how widely the surge rippled, not evidence of anything more.

Investor Takeaway

Most analysts set targets below the closing price even after upgrading, which signals that the science convinced Wall Street while the one-day move ran ahead of the fundamentals as they stand today.