Biotechnology company Moderna, one of the leaders in the race for a coronavirus vaccine, announced it would file Monday for regulatory clearance - a critical milestone that brings the United States a step closer to having two coronavirus vaccines by year's end.

Moderna's vaccine was 94% effective at preventing illness in a 30,000-person clinical trial, the company said - a performance that exceeds expectations and is on par with the best pediatric vaccines. All 30 cases of severe covid-19, the illness caused by the virus, were in a group that received a placebo.

The application arrives as public health experts gird for a blitz of coronavirus cases seeded by holiday travels and gatherings - a surge coming so soon that no vaccine can blunt it. But Moderna's filing marks the fourth Monday in a row with good news about the vaccine effort and means the United States could have enough vaccine to treat 20 million people by year's end, between doses of Moderna's vaccine and those of another candidate that is about a week ahead from Pfizer and German firm BioNTech. The Food and Drug Administration could authorize the vaccines for emergency use by mid- to late December.

"You don't want to get ahead of yourself and claim any victories, but this has the makings of a very, very important positive impact on ending this outbreak," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "A vaccine that is highly efficacious, if taken by a very, very high percentage of people, could potentially crush this outbreak - similar to what was done with outbreaks of measles and polio and smallpox and other diseases."

The coming weeks, while likely to strain an already overburdened health-care system, will also bring a whirlwind of regulatory decisions on the vaccines that could signal an eventual end to the pandemic.

"The data are very, very promising, but I would like to see more data than is currently in the press release," said Walter Orenstein, associate director of the Emory University Vaccine Center, who is also a member of Moderna's scientific advisory board and a trial participant.

Orenstein added that even those who are first in line to receive two doses of the vaccine will likely be encouraged to continue to take precautions, such as wearing masks, if virus is circulating at high levels in the community.

The vaccines from Moderna and Pfizer and BioNTech are a major proof of concept for a flexible and fast medical technology, years in the making, that utilizes a snippet of genetic material called messenger RNA that teaches cells to build the spiky protein found on the surface of the coronavirus. The immune system learns to recognize and block the real virus.

The data have yet to be published, but they will be scrutinized closely by regulators and an outside committee of experts in coming weeks. On Tuesday, an advisory committee to the Centers for Disease Control and Prevention will make recommendations on which groups of people should receive the first doses, which will be in short supply for months. Pfizer and BioNTech's vaccine will be debated by a group of expert advisers to the FDA on Dec. 10, and Moderna's vaccine is expected to be considered a week later, on Dec. 17.

The United States government, through Operation Warp Speed, has preordered 100 million doses from both Moderna and Pfizer, and helped underwrite the research and development of Moderna's vaccine to the tune of $955 million. Moderna also signed a contract to provide 100 million doses to the U.S. at $1.5 billion. Pfizer will provide 100 million doses at a cost of $1.95 billion. In addition, the government has an option to buy hundreds of millions more doses.

"Pfizer and Moderna's incredibly promising and impressive efficacy data further demonstrate that President Trump's Operation Warp Speed is rapidly advancing on a trajectory of success to save millions of American lives - five times faster than any other vaccine in history," White House spokesman Michael Bars said in an email.

Both companies have said they would be ready to distribute a vaccine almost immediately after receiving a green light from the FDA. Results reported last week from a third coronavirus vaccine candidate, from AstraZeneca and the University of Oxford, were positive but confusing, and it is unclear yet whether that vaccine can move forward in the United States without more data.

In Moderna's trial, volunteers were randomly assigned to two groups. One received two doses of the real vaccine and the other received doses containing salt water. Trial investigators then waited as people were exposed to the virus in their daily lives, to see if more cases occurred in the placebo group.

There were 196 cases of covid-19 in the study, 11 of which occurred in the vaccine group - a decisive signal that the vaccine protected people from illness. The 30 severe cases of covid-19 in the trial, including one death, all occurred in the group that got salt water shots.

The data will likely leave some questions unanswered. No one knows yet how long immune protection will last, and it is unclear whether the vaccine will decrease transmission in addition to preventing illness. One worst-case scenario that has been debated by scientists is a vaccine that prevents symptoms and disease but doesn't decrease the spread of the virus by asymptomatic people.

The next challenges will be distributing vaccines and communicating the logic behind prioritizing certain groups for the limited initial supply of vaccine. The Advisory Committee on Immunization Practices will take up this question Tuesday, and is expected to include health care workers in the first phase, a strategy intended to ensure the first doses have an important societal impact.

"There's a reason prioritization is set up as it is, and prioritizing health care workers first is also to keep our health care system functioning and to protect them," said Natalie Dean, a biostatistician at the University of Florida. "Absenteeism is a problem."

Health systems are already beginning to plan the rollout of potential vaccines to their front-line staff. Dean said her husband, a physician, recently received an email to register for a vaccine when one becomes available.

In its release Monday, Moderna reported that the vaccine performance was consistent across all ages, races, ethnicities and genders. There were 33 cases of covid-19 in people over 65; 29 cases in people who identified as Hispanic; 6 cases in Black people; 4 cases among Asian Americans; and 3 in multiracial people.

Most reactions to the vaccine were mild or moderate, according to a previous news release from the company. The adverse events rated as "severe" in the trial were soreness at the injection site, tiredness, muscle aches, headache and pain.

"We believe that our vaccine will provide a new and powerful tool that may change the course of this pandemic and help prevent severe disease, hospitalizations and death," Stéphane Bancel, Moderna's chief executive, said in a statement.

Moderna will also file Monday for a green light from the European Medicines Agency and has an open application with regulators in the United Kingdom, Switzerland, Canada, Israel and Singapore.

The vaccine can be stored frozen at minus-20 degrees Celsius and lasts at refrigerator temperatures for up to a month, which would likely make it easier to deploy than the candidate from Pfizer and BioNTech, which requires ultracold storage conditions not typically found at pharmacies and doctors' offices.