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At 6:45 a.m. Monday, a volunteer in Savannah, Ga., received a shot in the arm and became the first participant in a massive human experiment that will test the effectiveness of an experimental coronavirus vaccine candidate. The vaccine is being developed by the biotechnology company Moderna in collaboration with the National Institutes of Health.

The vaccination marks a much-anticipated milestone: the official launch of the first in a series of large U.S. clinical trials that will each test experimental vaccines in 30,000 participants, half receiving the shot and half receiving a placebo. Pharmaceutical giant Pfizer also announced it was initiating a 30,000-person vaccine trial, at 120 sites globally.

“We are participating today in the launching of a truly historic event in the history of vaccinology,” Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said at a news conference. He noted that the United States has never moved faster to develop a vaccine, from basic science to a large Phase 3 trial designed to test safety and effectiveness.

Fauci predicted that researchers would probably be able to tell whether the Moderna vaccine was effective by November or December, although he explained that it was a “distinct possibility” an answer could come sooner. Pfizer officials have said the company expects to be able to seek regulatory authorization or approval by October.

Company and government officials repeatedly underscored that although the vaccine development effort is moving at record-breaking speed, safety is not being sacrificed.

“There is no compromise at all, with regard to safety, nor of scientific integrity,” Fauci said.

Both vaccines require two doses, spaced several weeks apart. Then researchers will have to wait to see whether people get infected or sick from the novel coronavirus. What they hope to witness is a clear benefit: fewer infections in people who received the vaccine, or less severe episodes of covid-19, the disease caused by the coronavirus. There are many unknowns about how long it could take to see a clear signal of success or failure — including how fast the trials will recruit participants and how long it takes for enough people to become infected to observe whether there is an effect.

Statisticians have been crunching the numbers to predict how many infections would need to occur in the study population to gauge the vaccine’s effectiveness. To show the Moderna vaccine is 60 percent effective, Fauci said, there would need to be about 150 infections among the 30,000 participants.

The trials are also the biggest test yet of a promising technology that has never been approved for use outside medical research. Either vaccine could become the first in a new class of medicines. The vaccines deliver a snip of genetic material that carries the blueprint for the spiky protein that dots the surface of the coronavirus. After a person is vaccinated, their cells will follow the genetic instructions to build the proteins, and their immune systems, confronted with the spike protein, learn how to recognize and mount a defense to the virus without ever being infected.

“I believe it is a historic day: the first Phase 3 covid-19 vaccine being run in the U.S.,” Moderna chief executive Stéphane Bancel said. “It’s a historic day for science, as well. This is the first Phase 3 of a messenger RNA medicine in the world.”

Mark Mulligan, director of the NYU Langone Vaccine Center, said eight people will be vaccinated in the late-stage Pfizer trial Tuesday, after promising results in early-stage human tests.

“Now it becomes important to continue to assess them in larger numbers of people, and to ask the final question: Does it provide the protection?” Mulligan said.

Matt Slovick, 61, volunteered to be part of that history and showed up to receive a shot Monday afternoon at Meridian Clinical Research in Rockville, Md. Before the pandemic, Slovick, who works for an insurance company, did much of his work face-to-face, with on-site visits to clients and presentations to groups of people. Now, he works remotely and has seen small-business owners who are among his clients shut down. His oldest daughter was furloughed from her hospitality job during the pandemic, and his younger daughter was on the USS Theodore Roosevelt, the Navy aircraft carrier that was home to a major outbreak in March.

“Thank goodness, my daughter’s results came back negative,” Slovick said. “As an American, I was doing what I was supposed to do — staying at home, wearing a mask. I thought: Maybe I can help the whole populace of the country to get this thing going” when he heard about the vaccine trial.

Meridian Clinical Research in Rockville is one of 87 sites recruiting participants throughout the country for the Moderna trial — and was scheduled to vaccinate the first dozen people on Monday. Shishir Khetan, a physician leading the effort to recruit 300 to 400 people there, said the first day of any trial is typically slower than later days, but conducting a trial in a global pandemic is even more complicated. Researchers cannot conduct information sessions about the trial with groups, as they might under normal circumstances, or let people wait in a communal waiting room after vaccination.

Khetan said the biggest misconception he hears about the vaccine trial is the worry that the vaccine could infect people. But the vaccine doesn’t pose an infection risk; it’s just a fragment of genetic material that codes for a piece of the virus. He said he also encounters people who mistakenly believe trial participants will be infected with the virus.

“That’s absolutely not true. Nobody is given the virus,” Khetan said. “You’re encouraged to follow CDC guidelines of wearing a mask and social distancing.”

At least three other large trials facilitated by Operation Warp Speed, the federal effort to speed vaccine development, are expected to follow. Those include an experimental vaccine being developed jointly by the University of Oxford and AstraZeneca, one from Johnson & Johnson, and another candidate from the biotechnology company Novavax.

Interest in the trials is surging in the pandemic, but researchers said it would be essential for volunteers to include those who are most at risk of severe consequences of covid-19, including black, Hispanic, Native American and older people.

“This is going to be a big American opportunity for people to come onboard as our partners, to take part in what is a historic effort to bring to an end what has been the worst pandemic our world has seen in over 100 years,” National Institutes of Health Director Francis Collins said.

Moderna is planning to produce 500 million vaccine doses a year, with the possibility of making 1 billion doses annually in 2021. Over the weekend, the U.S. government committed $472 million to support the large trial, doubling the federal investment in Moderna’s vaccine candidate.

President Trump on Monday visited Fujifilm Diosynth Biotechnologies in North Carolina, a contract development and manufacturing organization working to ramp up production of the vaccine candidate being developed by Novavax. The Department of Health and Human Services also announced it was ramping up the nation’s ability to manufacture a vaccine by reserving capacity at the Texas A&M University System’s Center for Innovation in Advanced Development and Manufacturing through December of next year.

Several other vaccine developers have begun large trials designed to test effectiveness, including two candidates from Chinese companies and one being developed by the University of Oxford and AstraZeneca that is being tested in Brazil and South Africa and will soon start U.S. trials.