The arrival of the Omicron variant of SARS-CoV-2 in late 2021 marked a major shift during the COVID-19 pandemic, as the virus acquired dozens of mutations in its spike protein compared with earlier variants. Now, an epidemiological study in Nature shows that it also led to a shift in the protective effect of natural infection against reinfection: pre-Omicron, infection provided stable and durable protection, whereas infection during the Omicron era provided protection for only a short time. The authors conclude that this reflects a shift in evolutionary pressure from intrinsic transmissibility to immune escape. Moreover, the finding highlights the importance of regular updates of COVID-19 vaccines.