理解有误。参加三期的，定期检测病毒。所报道的病例数，是阳性者, 不仅仅是有症状者。请去读临试程序设计

https://www.niaid.nih.gov/news-events/phase-3-clinical-trial-investigational-vaccine-covid-19-begins

Volunteers must provide informed consent to participate in the trial. They will be asked to provide a nasopharyngeal swab and a blood sample at an initial screening visit and additional blood samples at specified time points after each vaccination and over the two years following the second vaccination. Scientists will examine blood samples in the laboratory to detect and quantify immune responses to SARS-CoV-2. 

Investigators will closely monitor participant safety. They will call participants after each vaccination to discuss any symptoms and will provide participants with a diary to record symptoms and a thermometer for temperature readings. 

If a participant is suspected to have COVID-19, the participant will be asked to provide a nasal swab for testing within 72 hours. If the test is positive for SARS-CoV-2 infection, the participant will be followed closely and referred for medical care if symptoms worsen. Participants will be asked to provide a daily assessment of symptoms through resolution and have saliva sampled periodically, so investigators can test for SARS-CoV-2 infection.