This note outlines a simple method for estimating the spread of the COVID 19 virus in the absence of data on test results for a large, random sample of the population. It applies the method to the UK, and other countries, and finds that to match data on daily new cases of the virus, the estimated model favours high values for the number of people infected but asymptomatic. That result is very sensitive to whether the transmission rate of the virus is di erent for symptomatic and asymptomatic cases, something about which there is significant uncertainty. This illustrates how di icult it is to estimate the spread of the virus until very large samples of the population can be tested.