U.S. biotech firm Moderna's experimental vaccine for COVID-19 reportedly showed promising results in the early stage of human trials.

Reuters said that in the test results published on Tuesday in the New England Journal of Medicine, the vaccine provoked immune responses in all 45 healthy volunteers in the early stages of the study.

No volunteers experienced serious side effects, but more than half reported mild or moderate reactions such as fatigue, headache, chills, muscle aches or pain at the injection site. These were more likely to occur after the second dose and in people who got the highest dose.

Moderna was the first to start human testing of a vaccine for the coronavirus on March 16.

The firm plans to enter Phase 3, the final stage of human trials for the vaccine, on July 27, recruiting 30-thousand participants in the U.S.