The research, led by Minnesota-based HealthPartners Institute, was published Thursday.
The plan is to provide booster shots to Americans beginning the Week of Sept. 20.
A booster shot would serve as a third dose of the Moderna or Pfizer-BioNTech vaccine.
The study found that vaccines are effective at preventing severe disease, but perhaps not as strong at infection prevention.
None of the trial participants that received the vaccine were diagnosed with the virus.
The drugmaker still needs federal authorization following more studies.
"Several hundred people" scheduled to get their first dose this week have seen their appointments be canceled.
The clinic is also open to veterans who are 65 years old and older.
He made his first court appearance on Monday.