The first phase of a US vaccine trial saw participants successfully generate antibodies for Covid-19 without serious adverse effects, according to results released on Tuesday.
The experimental vaccine, known as mRNA-1273, is being produced by the pharmaceutical company Moderna in partnership with the US government.
Injections of the trial vaccine were administered to 45 participants aged 18 to 55 starting in March. Most received two injections, with some receiving higher doses.
The regimen prompted "high levels of neutralizing antibody activity" that were above the average values seen in samples received from those who have recovered from Covid-19, the US National Institute of Allergy and Infectious Diseases said.
In April, the phase-one study was expanded to include adults over the age of 55. Those results have yet to be released.
Test 30,000 people
A phase-two clinical trial, sponsored by Moderna, began enrolment in late May, with plans to test 30,000 people in a phase-three trial later this month.
Moderna said the trial vaccine was "generally safe and well-tolerated, with no serious adverse events reported through Day 57."
"Adverse events were generally transient and mild to moderate in severity," Moderna added.