Competing interests: We have read and understood BMJ policy on declaration of interests and declare the following interests: JWT has given talks on general aspects of covid-19 (including transmission) at meetings sponsored by Thea Pharmaceuticals, Thornton & Ross, Landsec, and is conducting a study funded by Sanofi Pasteur on the effect of timing of seasonal influenza vaccination in healthcare workers on their vaccine induced immunity. LCM does consultancy for CrossFit, MITRE Corporation, and Smiths Detection. Provenance and peer review: Commissioned; not externally peer reviewed.