The company tested its vaccine on 10 adults between the ages of 56 and 70 and 10 elderly adults aged 71 and older, Moderna said. Each participant received two 100 microgram doses of the vaccine 28 days apart.

The volunteers produced neutralizing antibodies, which researchers believe are necessary to build immunity to the virus, and killer T-cells, Moderna said in its results, which have not yet been published in a peer-reviewed journal. Additionally, the antibodies that were produced were higher than those seen in people who have recovered from Covid-19.

The vaccine also appeared to be well tolerated, with no serious adverse events reported, the company said. Some patients reported fatigue, chills, headaches and pain at the injection site, though the majority of symptoms resolved within two days, the company said.