J&J scientists randomly assigned healthy adults between the ages of 18 and 55 and those 65 and older to receive a high or low dose of its vaccine — called Ad26.COV2.S — or a placebo. Some participants in the 18-to-55 age group were also selected to receive a second dose of the vaccine.

Most of the volunteers produced detectable neutralizing antibodies, which researchers believe play an important role in defending cells against the virus, after 28 days, according to the trial data. By day 57, all volunteers had detectable antibodies, regardless of vaccine dose or age group, and remained stable for at least 71 days in the 18-to-55 age group.

The most common side effects were fever, fatigue, headache, muscle aches and pain at the injection site, according to the trial data. Side effects were less common in the older age group, who received only one dose of the vaccine, as well as those who received a lower dose of the vaccine, according to the data.