Covid vaccines made Moderna a household name, but the company’s fortunes cratered as demand for the shots plummeted. Hostility from the Trump administration also spooked investors. As of Tuesday, the company’s stock had fallen nearly 90 percent from its peak, a loss of $170 billion in market value.

All the while, the company was quietly mounting a comeback in cancer.

On Wednesday, those efforts began to pay off in a big way. Moderna and the big drugmaker Merck reported that their experimental cancer vaccine extended the time before melanoma recurred in a clinical trial — the first late-stage study of its kind to succeed. The two drugmakers announced their success in news releases, without providing numbers that would indicate how much patients had benefited. Moderna’s stock closed up 177 percent on Wednesday.

The investors were betting that Moderna could help jump-start a new approach to treating certain cancers. The experimental vaccine uses the same messenger RNA, or mRNA, technology that powered Moderna’s Covid-19 shot, but it works differently to treat cancer. Tailored to each patient’s tumor genetics, the vaccine delivers instructions to produce a fragment of a tumor to teach the body’s immune system to attack the cancer.

Cancer vaccines using mRNA could become a hugely lucrative business, if the approach works across cancer types. Cutting-edge cancer drugs typically cost several hundred thousand dollars a year.

For Moderna, cancer “is by far the most significant pipeline opportunity,” said Tyler Van Buren, an analyst at the Wall Street bank TD Cowen.

Moderna’s chief executive, Stéphane Bancel, said on CNBC on Wednesday that the company’s work on the cancer vaccine was its latest effort to “take very big, scientific clinical bets.”

The prospect of using mRNA vaccines to treat cancer has tantalized researchers for decades. But momentum in the field has accelerated only recently, as manufacturing and other technology has improved, thanks in part to investments during the pandemic.

As of last year, more than 60 cancer vaccines using mRNA or similar technology were in development, being tested in more than 120 clinical trials, according to one count. Many of those studies are funded by philanthropies, the government and smaller biotechnology companies.

Four major drugmakers are leading the development efforts. Two are Moderna and Merck, whose collaboration on the approach dates back a decade; they are also testing the same vaccine as a therapy for cancers of the lung, kidney and bladder. (In the study whose results were announced on Wednesday, the cancer vaccine was given in combination with Keytruda, Merck’s blockbuster cancer immunotherapy.)

BioNTech, another developer of an mRNA vaccine for Covid, is working with Roche’s Genentech unit to develop a vaccine to treat cancers of the pancreas and colon. Results from a small safety study of that therapy in pancreatic cancer generated excitement this spring.

The approach has historically been seen as risky. The manufacturing is complex, the politics are fraught and there had been little evidence it would work. Other large major cancer drugmakers, like Pfizer and Bristol Myers Squibb, are not working on mRNA vaccines for cancer.

Under Health Secretary Robert F. Kennedy Jr., the Trump administration has made a series of funding and policy changes hostile to mRNA technology. A year ago, the federal government canceled nearly half a billion dollars’ worth of contracts and other funding for mRNA technology.

“Those political headwinds were serious, and it caused a lot of companies to take great pause,” said Jeff Coller, a scientist who works on mRNA at Johns Hopkins University. Dr. Coller advises several small mRNA companies and is on the executive committee of the Alliance for mRNA Medicines, a trade group.

But the administration has signaled it may be more open when it comes to mRNA for treating cancer. This year, it quietly started a public-private partnership expected to fund clinical trials of different cancer vaccines, including those using mRNA. The Department of Health and Human Services did not return a request for comment on Wednesday about the status of that initiative and how much funding the department has granted.

Moderna was founded in 2010 in Cambridge, Mass., to develop medicines using mRNA. Its Covid vaccine, its first product, brought in tens of billions of dollars of revenue during the pandemic.

But as the public and the government soured on its Covid shot, Moderna had hardly anything to replace it with. Over the past few years, Moderna looked for different ways to reinvent itself. It won approvals for mRNA vaccines for respiratory syncytial virus and, most recently, flu. It also explored experimental therapies for rare diseases.

Last year, the company laid off more than 800 workers, a tenth of its work force. It also lost more than $700 million in contracts to develop a shot to protect humans against bird flu after the Trump administration canceled the agreements. The company shelved vaccines to protect against herpes, chickenpox and shingles.

Even though it had started years before the pandemic, Moderna’s work on mRNA cancer vaccines flew relatively under the radar. In addition to the experimental cancer vaccine it is developing with Merck, Moderna solely owns several others that it is testing in different cancers.

“People started to pay attention” as Moderna and Merck began presenting promising results from midstage clinical research, said Andrew Tsai, an analyst at the Wall Street bank Jefferies.

Now, Moderna is back in the spotlight. Crucial questions remain: Will the promising results in melanoma, which has repeatedly proved to be more responsive to treatment, hold up in cancers that are tougher to treat? And will the detailed results look as good as the companies made them sound on Wednesday? (The drugmakers said they would present the full data soon at a medical meeting.)

But now that an mRNA cancer vaccine appears to have succeeded, Mr. Van Buren of TD Cowen said he anticipated that drugmakers would flock into the technology.

“Now that this trial was successful,” he said, “I definitely think we can expect more investment by other pharma companies.”