By Peter Loftus

New details about the first human study of Moderna Inc.'s experimental coronavirus vaccine emerged Tuesday, which researchers said reinforced their decision to take the shot into a large, decisive clinical trial scheduled to start in late July.

The new results, published online by the New England Journal of Medicine, showed that the vaccine induced the desired immune response for all 45 people evaluated -- a larger group than in the preliminary data Moderna released in May -- and was generally safe and well-tolerated.

"This is really quite good news," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in an interview Tuesday. NIAID co-developed the Moderna vaccine and led the study.

"The gold standard of protection against a viral infection is neutralizing antibodies," he added. "And the data from the study, small numbers as it may be, are pretty clear that this vaccine is capable of inducing quite good [levels] of neutralizing antibodies."

Researchers said they found no serious safety risks, though some participants had injection-site pain and symptoms such as fatigue, headache and chills.

Dr. Fauci said it is possible that the coming large study would yield an answer by year-end about whether the vaccine induced immune responses sufficient to protect people safely from Covid-19. A positive answer would clear the way for wider use and potentially help curb the deadly pandemic.

The new "Cove" study, scheduled to start July 27, will aim to enroll about 30,000 adults at nearly 90 different U.S. locations. Many of the study sites will be in states where the virus is surging, such as Texas, Florida, California and Arizona, according to information posted Tuesday on a federal database of medical studies.

Among the people researchers plan to enroll are those at appreciable risk of infection because of where they live. Testing the vaccine in coronavirus hot spots could help generate answers about the vaccine's efficacy sooner, because people there are more likely to be exposed to the virus in everyday living compared with people in places where new cases have declined.

Researchers will give about half of the participants the Moderna vaccine -- two doses four weeks apart -- and the others placebo shots, then compare what portion of each group contracts symptomatic Covid-19, the disease caused by the new coronavirus. The study is scheduled to run through October 2022, but it is likely researchers would have preliminary results well before then.

Moderna said Tuesday it has made enough doses to start the new study and is on track to deliver about 500 million doses a year, and possibly up to one billion a year starting in 2021.

Testing of the Moderna shot is one of a series of large, federally funded clinical trials planned in coming weeks of experimental coronavirus vaccines, including one codeveloped by the University of Oxford and AstraZeneca Plc, and another from Johnson & Johnson. Planning to start a large trial this month of their experimental coronavirus vaccine are Pfizer Inc. and BioNTech SE.

Some of the vaccine developers, including Moderna, say if studies are successful, a vaccine could become available for emergency use by the end of this year. Public-health officials say it is likely that several vaccines would be needed to help bring the deadly global pandemic under control.

Researchers are trying to line up tens of thousands of participants for the studies, recruiting at pharmacies, enlisting churches in searches for subjects and even requesting employees and families to ask around.

Last week a government-funded group of researchers leading the studies, the Covid-19 Prevention Network, started an online screening registry that would find and route potential volunteers to the various studies.

Since the online registry went live, about 62,000 people have signed up, according to a spokeswoman for Fred Hutchinson Cancer Research Center in Seattle, which is helping to lead the study network.

Larry Corey, a Fred Hutchinson vaccine specialist who is helping to lead the effort, said the initial number of registrations was encouraging, though it is possible that not all the volunteers would end up in the trials.

Dr. Corey said researchers are looking for diverse study populations that include people at risk for severe disease because of their age, ethnicity and race. Older people are more vulnerable to the virus, and rates of hospitalization and death are higher in minority groups.

The National Institute of Allergy and Infectious Diseases, which codesigned a coronavirus vaccine with Moderna, of Cambridge, Mass., started the first study of the shot in March, after Moderna quickly manufactured the first batch. The vaccine is designed to deliver the genetic code of a portion of the new coronavirus, which instructs the body's cells to make that protein and in turn induce an immune response against the virus.

In May, Moderna said preliminary results showed that the vaccine induced desired immune responses in a small number of people and was generally safe and well-tolerated.

The new results are from 45 people, ages 18 to 55, who enrolled in the study in Seattle and Atlanta. The study was later expanded to include people older than 55, but they weren't included in the new analysis.

Study subjects received two shots, about four weeks apart. Researchers tracked safety and whether the vaccine induced certain immune responses as measured in blood samples.

Researchers found antibody production generally increased after the second dose. They found neutralizing activity similar to that seen in the top half of blood specimens taken from 41 people who had recovered from Covid-19 and who weren't in the vaccine study.

The neutralizing activity was detected up to nearly two months after the first injection, a longer follow-up period for more people than was available at the time Moderna released preliminary results in May, said Dr. Lisa Jackson, a senior investigator at Kaiser Permanente Washington Health Research Institute in Seattle, and lead author of the study.

Researchers, however, still don't know what level of a neutralizing immune response would be needed to guard against either infection or severe disease, and for how long a vaccine could provide such protection.

"This provides some essential first information on how the vaccine is tolerated and how the doses affect immunity, and I think it's essential information to progress onward," Dr. Jackson said.

More than half of the study participants had side effects including fatigue, chills, headache and injection-site pain. Three participants who received the highest dose level tested had one or more severe adverse events, the researchers said. That highest dose level isn't being tested in the large phase 3 study.

Write to Peter Loftus at peter.loftus@wsj.com