"Deeks says that a December trial at the University of Birmingham is an example of how rapid tests can miss infections. More than 7,000 symptom-free students there took an Innova test; only 2 tested positive. But when the university researchers rechecked 10% of the negative samples using PCR, they found another 6 infected students. Scaling that up across all the samples, the test probably missed 60 infected students. Mina says that these students had lower levels of virus, so were unlikely to be infectious anyway. Deeks argues that although people with lower virus levels might be in the late phase of a waning infection, they might also be on the way to becoming more infectious. Another factor is that some students might have done a poor job taking swab samples, so that not many viral particles made it to the test. He’s worried that people will falsely think they’re safe with a single negative test — when in fact a rapid test is only a snapshot of likely non-infectiousness at that moment. Remarks that the tests could make workplaces completely safe are not the right way to inform the public about their efficacy, Deeks says. “If people get a false sense of security, they could actually spread the virus around,” he says."