A vaccine to prevent COVID-19 may be available early next year if treatments now being tested prove effective, medical experts believe.

Moderna Inc said its coronavirus vaccine, mRNA-1273, produced neutralizing antibodies in 45 patients who participated in an early-stage human trial, according to data published in The New England Journal of Medicine.

"Phase 1 data demonstrate that vaccination with mRNA-1273 elicits a robust immune response across all dose levels," said Tal Zaks, Moderna's chief medical officer, in a statement. "We look forward to beginning our Phase 3 study this month to demonstrate our vaccine's ability to significantly reduce the risk of COVID-19 disease."

Participants in the study group received 25-, 100- or 250-microgram doses of the vaccine. There were 15 people in each study group, and participants received two doses each, the company said.

After two shots, the vaccine produced a "robust" immune response in all cases. Participants receiving the highest dose of the experimental treatment produced antibodies four times higher than levels found in patients who have recovered from the coronavirus, Moderna said.

Most trial participants tolerated the vaccine, but about half reported mild to moderate symptoms including fatigue, muscle aches or pain at the injection site, the company said.

Moderna said it plans to begin a late-stage trial of the vaccine July 27. It will include 30,000 participants in 87 locations. Each will receive a 100-microgram dose or a placebo and a second shot 28 days later, according to clinicaltrials.gov.

In May, Moderna released preliminary information derived from an early-stage trial, but that data hadn't been reviewed by other experts.

"This should further increase confidence that we are getting a robust immune response, in that there should be greater confidence that this will be protective to a degree in transmission of COVID," said Michael Yee, a managing director at Jefferies Financial Group, to CNBC.

"This is all along our positive thesis and our view that both Moderna and Pfizer-BioNTech are definitely on a good track to get a vaccine by the end of the year."

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, has said he's "cautiously optimistic" researchers will develop at least one effective vaccine by the end of this year or early 2021.

US officials hope to deliver about 300 million doses of a coronavirus vaccine by early 2021.

More than 135 vaccines are under development by drug companies throughout the world, according to The New York Times, but only four have gotten to Phase 3 testing.