Researchers also reported some side effects in the 45 people in the phase I study, but no significant safety issues, the news release says.
The vaccine is among hundreds being tested worldwide in an effort to halt the pandemic that has killed nearly 600,000 worldwide.
A researcher testing the vaccine called the results encouraging but cautioned more study is needed. “Importantly, the vaccine resulted in a robust immune response,” Evan Anderson, MD, principal investigator for the trial at Emory University, says in a news release. Emory and Kaiser Permanente Washington Health Research Institute were the two sites for the study.
The company is already testing the vaccine in a larger group of people, known as a phase II trial. It plans to begin phase III trials in late July. Phase III trials involve testing the vaccine on an even larger group and are the final step before FDA approval.
The study results are published in The New England Journal of Medicine. The study was led by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health.
Moderna's vaccine uses messenger RNA, also called mRNA. It carries the instruction for making the spike protein, a key protein on the surface of the virus that allows it to enter cells when a person is infected. After it’s injected, it goes to the immune cells and instructs them to make copies of the spike protein, acting as if the cells have been infected with the actual coronavirus. This allows other immune cells to develop immunity.
In the study, participants were divided into three groups of 15 people each. All groups received two vaccinations 28 days apart. Each group received a different strength of the vaccine -- either 25, 100, or 250 micrograms.
Every person in the study developed antibodies that can block the infection. Most commonly reported side effects after the second vaccination in the 100-microgram group were fatigue, chills, headache, and muscle pains, ranging from mild to moderately severe.
The phase II study has 300 heathy adults ages 18-55, along with another 300 ages 55 and older
Moderna says it hopes to include about 30,000 participants at the 100-microgram dose level in the U.S. for the phase III trial. The estimated start date is July 27.