Biotechnology firm Moderna announced Monday that a preliminary analysis shows its experimental coronavirus vaccine is nearly 95 percent effective at preventing illness, including severe cases — a striking initial result that leaves the United States with the prospect that two coronavirus vaccines could be available on a limited basis by the end of the year.

The news comes a week after pharmaceutical giant Pfizer and its German partner BioNTech lifted the stock market and people’s hopes with the news that their coronavirus vaccine was more than 90 percent effective. The announcement sent stocks up again, with the Dow Jones industrial average and S&P 500 up about 1 percent in midmorning trading. Moderna’s share price rose more than 6 percent.

At a briefing Monday, government officials predicted that if the two vaccines receive a regulatory greenlight, the first shots could be given in December, with enough to vaccinate 20 million people that month — and more becoming available into 2021 as production ramps up and other vaccine candidates may be successful.

“It’s extremely good news. If you look at the data, the numbers speak for themselves,” said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, who was one of three people briefed on the data by an independent committee Sunday morning. “I describe myself as a realist, but I’m fundamentally a cautious optimist. I felt we’d likely get something less than this. … I said certainly a 90-plus-percent effective vaccine is possible, but I wasn’t counting on it.”

Moderna’s vaccine, co-developed with Fauci’s institute, is being tested in 30,000 people. Half received two doses of the vaccine, and half received a placebo. To test how well the vaccine works, physicians closely monitored cases of covid-19 to see whether they predominantly occurred in people who received the placebo.

Of the 95 cases of covid-19, the disease caused by the virus, 90 were in the group that received the placebo. There were 11 severe cases reported — all in people who received the placebo. With cases of covid-19 confined almost exclusively to trial participants receiving a placebo, that sends a strong signal that the vaccine is effective at thwarting the virus.

The data have not yet been published or peer reviewed, and the overall effectiveness of the vaccine may change as the study continues. But Fauci said the data on severe cases was “quite impressive” and effectively answers a question that has lingered: whether a vaccine measured by its success in preventing any case of covid-19 can prevent the most urgent cases, too.

An independent data committee, convened by the National Institutes of Health, analyzed the results Sunday morning. Stéphane Bancel, chief executive of Moderna, said in an interview that he spent the morning trying to distract himself from wondering about the results by working at his home in Boston, but instead he found himself constantly checking his phone and email. When he learned the results later in the morning, the evidence that the vaccine prevented severe disease stood out as most consequential.

“In this pandemic, what has been awful from a public health standpoint, an economic standpoint, is the worry people have to get so sick they have to go to the hospital — so sick they have to get to the ICU and have a high risk of dying,” Bancel said. “If a [vaccine] could prevent 95 percent of people to not get disease, but to not get severe disease, that would be a game-changer: the impact on hospitals, the impact on people’s psyche and the impact on deaths.”

Many questions about details remain. How long will the protection last? Will results be similar across all subgroups? Does the vaccine decrease the infectiousness of the virus in people without symptoms?

But the early successes show the power and speed of a new vaccine technology never before used in an approved medical product that delivers a strip of genetic material called messenger RNA to teach the body to defend against the virus. The messenger RNA carries the blueprint for the distinctive spike protein that dots the outside of the coronavirus, instructions that the body’s cells follow to build the spike protein.

“It is extremely encouraging,” said Stanley Plotkin, inventor of the rubella vaccine. “This and the earlier result shows that the platform really works, and this bodes very well for other diseases, where this platform could be used — and considering the speed with which the platform was put into operation. It’s an excellent result.”

Moderna has committed to completing its trial before applying for emergency-use authorization — which means waiting until there are 151 cases of covid-19 in the study. A previous projection showed that the trial might end sometime early next year, but it is instead expected to reach its endpoint in seven to 10 days, Bancel said, because of surging coronavirus cases in the United States. The explosion of virus cases translates into an expedited ability to ascertain whether a vaccine works.

The company will have enough safety data to support an application shortly before Thanksgiving. Bancel anticipates a vaccine might begin to become available to those at high risk in the second half of December.

Unlike Pfizer, which invested $2 billion of its own money in researching and developing a vaccine, Moderna is part of Operation Warp Speed, the government initiative designed to erase the financial risk of vaccine and therapeutics development by providing upfront funding to companies and helping coordinate the trials. Moderna received nearly $1 billion from the U.S. government to support research and development. The companies both have contracts to sell 100 million doses to the U.S. government.

Moderna projects it can produce 20 million doses by the end of the year — enough for 10 million people to get both shots. The company aims to produce at least 500 million doses next year, with the possibility of scaling up to 1 billion doses depending on the availability of raw materials.

The side effects of the two-dose vaccine were mostly mild or moderate, including pain at the injection site, fatigue, headache and muscle pain, according to the company’s news release.

Regulators at the Food and Drug Administration must review the evidence for the Moderna and Pfizer vaccines, but the robust early indication of success suggests both vaccines might become available to high-risk populations before the end of this year.

Fauci predicted that people such as health-care workers or people with conditions that raise their risk of developing severe disease could begin receiving doses before the end of the year. It could take about four months to vaccinate people in high-risk groups, and in April, the vaccine could expand to the rest of the population.

The two successes could have ripple effects that influence the ongoing trials and in future coronavirus vaccine trials. People currently in the Moderna and Pfizer trials do not know whether they received the real vaccine or placebo, and at some point, those who received a placebo may be crossed over to receive the real thing. But the decision to do that will mean a loss of follow-up and valuable data.

“Given these products will be given to potentially billions of people, you have to be careful to make sure they’re safe and effective, and there’s no easy way around that issue,” said Ira Longini, a biostatistics expert at the University of Florida, who said a year of follow-up on participants would be ideal.

The focus on vaccines will now shift to the daunting logistics of manufacturing and distribution.

The Pfizer vaccine requires ultracold storage conditions — minus 70 degrees Celsius — not widely available in typical vaccination settings. The company has been working to overcome that limitation.

Moderna announced Monday that its vaccine can be stable at refrigerator temperatures for a month and frozen for up to six months. It will not require dilution at the point of care, unlike the Pfizer vaccine.