The team conducted a phase 1, dose-escalation, open-label trial with 45 healthy adults, 18 to 55 years of age. Each adult received two vaccinations, 28 days apart, with mRNA-1273 in different dosages: 25 μg, 100 μg and 250 μg. There were 15 participants in each dose group.

The vaccine was administered as a 0.5-ml injection in the deltoid muscle on days 1 and 29 and follow up visits were scheduled after each vaccination. Normal saline was used as a diluent to prepare the doses administered.

The dose-escalation plan specified four sentinel participants to be enrolled in both the 25-μg group and the 100-μg group, followed by full enrollment of those two dose groups. Sentinel participants are dosed in advance of the full study and are monitored to ensure no adverse effects take place. If no halting rules were met after all participants in those two dose groups completed day 8, four sentinel participants in the 250-μg group were enrolled, followed by the remainder of that dose group.

Participants recorded reactions using a memory aid for 7 days after each vaccination and adverse events were graded according to a standard toxicity grading scale.