CHICAGO: Moderna Inc's experimental vaccine for COVID-19 showed it was safe and provoked immune responses in all 45 healthy volunteers in an ongoing early-stage study, U.S. researchers reported on Tuesday.

Volunteers who got two doses of the vaccine had high levels of virus-killing antibodies that exceeded the average levels seen in people who had recovered from COVID-19, the team reported in the New England Journal of Medicine.

No study volunteers experienced a serious side effect, 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.

Experts say a vaccine is needed to put an end to the coronavirus pandemic that has sickened millions and caused nearly 575,000 deaths worldwide.

Moderna was the first to start human testing of a vaccine for the novel coronavirus on March 16, 66 days after the genetic sequence of the virus was released.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, whose researchers developed Moderna's vaccine candidate, called the results "good news," noting that the study found no serious adverse events and the vaccine produced "reasonably high" levels of virus-killing or neutralizing antibodies.

"If your vaccine can induce a response comparable with natural infection, that's a winner," Fauci said in a telephone interview. "That's why we're very pleased by the results." Moderna shares jumped more than 15% in after-hours trading on Tuesday.