Those early volunteers developed neutralising antibodies in their bloodstream – which are key to blocking infection – at levels comparable to those found in people who survived Covid-19, the research team reported in the New England Journal of Medicine.
“This is an essential building block that is needed to move forward with the trials that could actually determine whether the vaccine does protect against infection,” said Dr Lisa Jackson, who led the study.
An experimental coronavirus vaccine has triggered immune responses and shown itself to be safe in early-stage human tests in the US, researchers have said.
The results have been hailed as “good news” by Dr Anthony Fauci, America’s leading infectious diseases expert.
On Tuesday, researchers reported the findings from the first 45 healthy adults who received Moderna Inc’s experimental vaccine in March, which showed it provided a hoped-for immune boost.
Dr Fauci, the director of the National Institute of Allergy and Infectious Diseases, whose researchers developed Moderna’s vaccine candidate, said: “If your vaccine can induce a response comparable with natural infection, that’s a winner.
“That’s why we’re very pleased by the results.”
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.
Moderna was the first to start human testing of a vaccine for the virus on 16 March.
In April, the phase 1 trial – which aims to ensure treatment is safe and helps determine an effective dose – was expanded to include adults over 55, who are more at risk of serious disease. Moderna said it would follow volunteers for a year to look for side effects and check how long immunity lasts.
The experimental vaccine will start its most important step around late July: a 30,000-person study to find out if the shots really are strong enough to protect against coronavirus.
Nearly two dozen possible Covid-19 vaccines are in various stages of testing around the world.
A University of Oxford potential vaccine is already in large-scale phase three human trials to assess whether it can protect against Covid-19, but its developers have yet to report phase 1 results which would show whether it is safe and whether or not it induces an immune response.
The developers of the vaccine said they were encouraged by the immune response they have seen in trials so far and were expecting to be able to publish phase 1 data by the end of July.
Researchers at Imperial College London started the UK’s second human trial into a coronavirus vaccine last month.