Question to the Department of Health and Social Care:

To ask Her Majesty's Government whether any risk models used on smart phone apps to perform contact tracing use Bayesian inference to decide the best course of action given the test results of the user and those of other users.

The National Health Service COVID-19 app’s risk scoring algorithm does not currently use Bayesian inference. Instead, a user’s risk is calculated using a combination of heuristics, developed in collaboration with expert epidemiologists from the University of Oxford’s Big Data Institute. We have published an explanation of how the risk scoring algorithm works on the FAQ page of the NHS COVID-19 website in an online only format. For future versions of the app, we are working on a fully probabilistic, Bayesian risk scoring algorithm.