An infected person produces respiratory droplets when talking, coughing and sneezing. These are responsible for the transmission of virus between people. Droplets can travel up to 2m, with finer aerosols containing smaller viral particles travelling even further. Numerous complex and interacting factors influence how they move and settle onto surfaces, and how infectious they are. The further away a person is, the fewer droplets they will be exposed to and so their risk of being infected with the virus reduces. The advice on 2 m distancing is a risk assessment based on relative not absolute risk; 2 m does not represent zero risk. Measures to mitigate the increased risk of reducing physical distancing include ventilation, physical barriers (screens and face coverings), reduced building occupancy and enhanced cleaning. These will vary according to the context. The wider range of social distancing practices will need to be maintained to contain viral transmission even if the 2 m advice changes. Social distancing and other public health measures are likely to be needed long-term, until a vaccine or more effective treatments for COVID-19 are available. There are numerous knowledge gaps about SARS-CoV-2 transmission; research to address them will inform policy-making.