The drug manufacturer Moderna Inc. announced dramatic early results for a second COVID-19 vaccine Monday, saying the drug appeared to reduce the chance of illness by 94.5%.

The findings come a week after Pfizer Inc. said its vaccine seemed to reduce the rate of disease by more than 90%. Both companies say that within weeks, they intend to seek U.S. government approval to distribute their products on an emergency basis to the broader public. Enough doses would be ready to administer to the first few million recipients by the end of the year, with hundreds of millions more on the way in 2021, government and company officials said Monday in a joint news briefing.

Moderna, based in Cambridge, Mass., had projected that it would need at least 151 cases of illness to tell if its vaccine was working, based on an initial estimate of 60% efficacy. But the results as of Monday, based on just 95 cases, were so clear-cut that the drug maker announced them sooner, as dictated by a protocol filed in advance with the U.S. Food and Drug Administration.

The 94.5% figure means that in the 95 cases so far, 90 cases of COVID-19 were identified among those who received a placebo, while just five cases occurred in those who got the actual vaccine. Those numbers represent just a small fraction of the 30,000 people who got either the drug or the fake version; the rest have not become ill, presumably because so many continue to take precautions against infection. As with the Pfizer vaccine, no serious side effects were reported.

Though preliminary, the results generally drew optimism from prominent infectious disease experts on social media, and from Anthony S. Fauci, head of the National Institute of Allergy and Infectious Diseases.

“The results of this trial are truly striking,” he said at the briefing.

What’s more, Moderna said its drug would be easier to store, not requiring the ultracold temperatures of the Pfizer product.

The Moderna vaccine can be stored for up to six months at an average of negative 4 degrees Fahrenheit, and it remains stable for up to 30 days at standard refrigerator temperature, the company said.

Pfizer has said its vaccine must be kept at an average temperature of negative 94 degrees Fahrenheit, requiring special freezers. Once doses are delivered to a hospital or other medical setting, they can be kept for up to five days at 18 to 28 degrees, within range of standard freezers, the company says.

Yet supply-chain logistics have been worked out for both drugs, said Alex Azar, secretary of the U.S. Department of Health and Human Services, who joined Fauci at the briefing. CVS and Walgreens are able to accommodate the “cold-chain” requirements of the Pfizer vaccine, while other pharmacies are equipped to handle the Moderna product, he said.

If the vaccines were approved next month, just one year will have elapsed since the coronavirus emerged in China — a record for developing such drugs. That speed was made possible by technological advances, including some that took place years ago at the University of Pennsylvania, but also because of ample government support from the Trump administration. Moderna got funds to support its research as part of Operation Warp Speed, while Azar’s agency has pledged money to Pfizer for manufacturing and distribution.

Joining Azar and Fauci at the briefing was Stéphane Bancel, Moderna’s chief executive officer.

“We are on track to make between 500 million and 1 billion doses in 2021,” he said.

Among the 95 patients represented in Moderna’s early results were 11 who suffered severe cases of COVID-19. All 11 received the placebo, not the actual vaccine, fueling hopes that the drug could prevent serious cases of the disease. Pfizer’s results, on the other hand, were not broken down by disease severity, which prompted some physicians to reserve judgment last week.

Another unanswered question: whether some people who get the vaccines will receive partial protection — meaning they could still become infected but suffer only mild symptoms or none at all. That outcome would be a plus — unless such people were unwittingly able to spread the virus, said John Zurlo, chief of the division of infectious diseases at Thomas Jefferson University.

Though Moderna’s trial was designed to look for any evidence of that problem, no details were disclosed in the company news release.

Still, the results are promising, Zurlo said. And like many other medical facilities, Jefferson University Hospital already is preparing to administer vaccines to its employees, with the help of detailed guidance from the Philadelphia Department of Public Health, he said.

Nurses, doctors, and other medical personnel will be among the first to get vaccines, both because their services are essential in a continuing pandemic and because they are at higher risk of exposure.

“We get vaccine on a Tuesday, we want to start vaccinating on a Wednesday,” Zurlo said.

Both the Moderna and Pfizer vaccines consist of RNA, the molecular cousin of DNA. They contain genetic instructions for a fragment of a coronavirus — one of the “spike” proteins that protrude from the surface of each virus particle. These instructions allow human cells to make the protein, which in turn prompts the immune system to develop customized antibodies and other defenses should it ever encounter a live infection.

In 2005, scientists at the University of Pennsylvania started laying the groundwork for this approach. The university licensed the fruits of that research years ago to two biotech firms, which in turn have sub-licensed it to both Moderna and to BioNTech SE, the German company that is working with Pfizer. That means Penn stands to make money if either vaccine is approved, though terms are confidential.

Dozens of vaccines are in development worldwide, though most have not yet reached the “phase 3” stage like Moderna and Pfizer, which administered their products to tens of thousands of volunteers. Among the other candidates is one made by Inovio Pharmaceuticals of Plymouth Meeting, which also reported progress Monday. The company announced that the FDA had cleared it to resume phase 2 tests in a smaller number of human volunteers.

As with the Pfizer announcement last week, investors cheered the Moderna news, driving up its stock price nearly 10% Monday. Inovio shares fared even better, rising more than 14%. Pfizer’s share price was down 3% on Monday, though it remains slightly higher than before the vaccine results were reported a week ago.