A COVID-19 vaccine candidate has shown it can prompt an immune response in the human body, and was also found to be safe and well-tolerated in a small group of patients.

Moderna, a Cambridge, Massachusetts-based company that manufactured the vaccine, announced the encouraging early results from its phase 1 clinical trial Monday morning. The drug is now being tested in larger studies.

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The phase 1 trial involved 45 people. Results on eight of those participants revealed they developed neutralizing antibodies — which are believed to be key in providing protection from the virus — at levels seen in patients who’ve recovered from COVID-19.

“We could not be happier about this interim data,” Moderna CEO Stéphane Bancel said during a conference call with investors Monday. He said the results indicate the vaccine has a “high probability to provide protection from COVID-19 disease in humans.”

The goal of phase 1 trials is to evaluate the safety of a new drug in a small group of participants, not effectiveness. Phase 2 trials study whether a drug works.

In total, 45 study participants each received two doses of the vaccine, about a month apart. Participants were given one of three dosage levels: 25 micrograms, 100 micrograms and 250 micrograms.

Moderna said those who received the two lower doses developed COVID-19 antibodies about two weeks after the second dose. Final information on the 250 microgram dosing was not available.

The only side effect, the company reported, was redness around the area where people got the shot.

Physicians not involved with the trials expressed optimism based on the study results, even though it’s still relatively early in the clinical trial process.

“It’s great news to see what appears to be a potentially effective vaccine has been created and appears safe,” Dr. Iahn Gonsenhauser, chief quality and patient safety officer at the Ohio State University Wexner Medical Center, said. What’s more, it’s promising the antibodies the patients developed were “shown to be effective at limiting the virus’s replication in the lab,” he added.

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Still, many questions remain about the level of protection those antibodies might offer, Dr. Michael Ison, a professor in the division of infectious disease and organ transplantation at Northwestern University Feinberg School of Medicine in Chicago, said.

It will be important to continue to monitor patients in the study because Moderna is using a kind of vaccine approach, called messenger RNA, that’s never been marketed in the United States, he said.

“Overall this looks pretty good,” Ison said. “That being said, this is a new kind of vaccine platform, and some of the safety signals may take time to be recognized.”

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In earlier mouse studies, the vaccine prevented the virus from reproducing in the animals’ lungs.

Moderna, which has partnered with the National Institute of Allergy and Infectious Diseases in the trials, is pushing forward with a larger phase 2 study. That phase will also look at the effectiveness of a 50 microgram dose.

The company expects to begin a phase 3 clinical trial as early as July. Phase 3 trials involve a much larger group of participants to confirm results of phases 1 and 2.

Other COVID-19 vaccine candidates are being studied at Oxford University in the United Kingdom and at a company called Inovio, headquartered in Pennsylvania.

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