The UK response to Covid-19 has been unusually complex in its ever-shifting classifications of clinical vulnerability. By May 2020, 2.2 million people had been identified as ‘clinically extremely vulnerable’ (CEV) and were asked to ‘shield’ at home for over four months. To adhere to this strict guidance, they were enfolded within the patchy infrastructure of the ‘shielding programme’. However, membership of the ‘shielded list’ has changed—often without warning or explanation—through time and across space. Drawing on policy and evidentiary documents, government speeches, reports, press conferences and media analysis of Covid-19 coverage between March 2020 and April 1, 2021, this paper traces the shifting delineations of clinical vulnerability in the UK response across three lockdowns. It argues that the complexities and confusions generated by the transience of the CEV category have fed into forms of biosociality that have been as much about making practical sense of government guidance as a form of mutual support amid crisis. This uncertainty has not eased as restrictions have been relaxed and vaccines rolled out. Instead, tracing individual immune response has become a burgeoning industry as ‘the shielded’ navigate the uneasy demands of taking ‘personal responsibility’ rather than being protected by ‘the rules’.