According to a new study, rapid antigen tests may provide false negatives — suggesting that a person is infection-free, even though other parts of their respiratory tract are teeming with the virus. The researchers found that most people showed a delay of several days between when the virus first appeared in the throat or saliva and when it appeared in the nose. Importantly, 15 of the 17 study participants had high levels of the virus for at least a day prior to getting a positive result.