Last week, testing at a private Covid lab in Wolverhampton was halted, after the UK Health Security Agency found tens of thousands of people may have been falsely given a negative PCR result. But since the start of September, scientists had been alerted to strange patterns in the testing data which suggested something was out of the ordinary. Anand Jagatia speaks to Dr Kit Yates, a mathematical biologist, about why it took so long for these errors to be traced back to the lab, and what the consequences could be. Help support our independent journalism at theguardian.com/sciencepod