The C.D.C. enrolled 3,950 people at high risk of being exposed to the virus because they were health care workers, first responders, or others on the front lines. None had previously been infected with the coronavirus.
Most — 62.8 percent — received both shots of the vaccine during the study period, and 12.1 percent had one shot. The participants had no previous infections with the coronavirus.
Participants collected their own nasal swabs each week, which were sent to a central location for P.C.R. testing, the most accurate type of test. The weekly swabs allowed the researchers to detect asymptomatic infections as well as symptomatic ones.
The investigators also asked participants about symptoms associated with infection, including fever, chills, cough, shortness of breath, sore throat, diarrhea, muscle aches, or loss of smell or taste. The researchers also analyzed patients’ medical records to detect illnesses.
Fifty-eight percent of the infections were detected before people had symptoms. Just 10.2 percent of infected people never developed symptoms.