Support theory suggests that the judged probability of events depends on the explicitness of their description. We tested whether risk communication messages that specify risks involved are associated with increased intentions to comply with public health advice during a pandemic. We conducted an anonymous online survey of the U.K. and U.S. public between April 24 and May 12, 2020. Participants (N = 2087) rated 14 COVID-related symptoms in terms of perceived severity and induced worry. They were then asked about their intention to practise social distancing in response to three public health messages: the standard U.K. government message: “Most people will experience only mild symptoms”; the standard message “unpacked” by listing six of those symptoms as examples; and “Most people will not require hospitalisation.” The unpacked message resulted in the highest intention to comply with social distancing (b = .22 [.04, .40], p = .02) and there was no interaction with country. Worry about symptoms was an independent predictor of intention to comply (b = .02 [.01, .03], p < .001). In the days before lockdown amidst a raging pandemic, the U.K. and U.S. governments sought to reassure the public. Had their messaging been more detailed, it might have been less reassuring but more effective in promoting social distancing.