The vaccine candidate was co-developed by researchers at Moderna and the U.S. National Institute of Allergy and Infectious Diseases (NIAID). The mRNA vaccine encodes a prefusion stabilized form of the SARS-CoV-2 spike protein.

The first clinical batch of vaccine, manufactured by the Coalition for Epidemic Preparedness Innovations (CEPI), was completed on February 24, just 42 days from sequence selection. The first participant in the NIAID-led phase I study was dosed on March 16.

The trial was conducted at Kaiser Permanente Washington Health Research Institute in Seattle, Emory University in Atlanta, and NIAID's Vaccine Research Center clinic at the National Institutes of Health Clinical Center in Bethesda, MD.

The study was expanded to enroll older adults about one month after initiation. Older adults are more vulnerable to complications with COVID-19 and are thus an important population for vaccination. The study enrolled 40 healthy volunteers including 20 adults ages 56-70 years, and 20 adults ages 71 years and older.

Ten volunteers each received a lower dose (25 µg) or higher dose (100 µg) of the vaccine. After approximately one month, volunteers received a second dose of the vaccine. Volunteers were tracked throughout to monitor responses and assess safety.

Both dose levels of the vaccine were well-tolerated in older adults. Immune responses were found to be dose-dependent with the 100-µg dose, eliciting higher binding and neutralizing antibody titers. Importantly, the immune responses to the vaccine observed in older volunteers were comparable to that seen in younger age groups.

A phase II clinical study of mRNA-1273 was initiated on May 8, with enrollment completed on July 8. Then the phase III Cove study of mRNA-1273 in collaboration with the Biomedical Advanced Research and Development Authority, a U.S. federal health agency, began on July 27 and is ongoing.