Imprinted hybrid immunity against XBB reinfection

In the fourth year of the COVID-19 pandemic, the picture of population immunity has become increasingly complex and the certainties have become less certain.1 Since the start of 2022, many countries have had sequential waves of infection with emerging subvariants of the SARS-CoV-2 omicron (B.1.1.529) variant. Clinical phenotypes associated with these waves in different settings have been diverse. In the USA and the UK, in which relatively high proportions of the population have received three or more doses of an mRNA vaccine, some large waves of breakthrough infections saw more than 8% of the population infected with SARS-CoV-2 (reported in weekly estimates of the rate of COVID-19 infections by the UK Office of National Statistics Coronavirus [COVID-19] Infection Survey), although sufficient protective immunity prevented a commensurate peak in deaths.