We don’t know how much Test and Trace affects the spread of Covid-1918 February 2021What was claimed In October, Test and Trace was reducing R by between 0.3 and 0.6. Over the past few weeks, the chair of NHS Test and Trace, Baroness Dido Harding, told two MPs’ committees that the programme significantly reduced the spread of Covid-19 last October. Untangling all the different effects in order to work out the true impact of Test and Trace would be extremely complicated. Even so, there is a lot more uncertainty in the estimate than Baroness Harding indicated in the select committee hearings. As a result, the true effect of Test and Trace in reducing the spread of Covid might be much less than she claimed.