There have been several studies (link) discussing rapid testing. Research confirms that rapid antigen tests miss omicron in the first few days after infection, when the viral load in the upper respiratory tract is sufficient to infect other people. The researchers observed workers who had to do both PCR and antigen tests every day. The authors could compare when both would catch the infection. Twenty-eight of the 30 sick workers tested negative for antigen, an average of three more days after PCR showed infection, and with a high viral load.Viral load peaks 1-2 days earlier in tests that use saliva as a sample than in tests that require a nasal swab. This also applies to PCR tests. Similar results have been previously obtained in other studies, so the conclusions of this study can be considered independent confirmation.
The delay in rapid tests further contributes to the spread of omicron, at least in Europe, where such tests are widely available. People who have received a negative antigen test continue to communicate and at the peak of the viral load continue to infect others.