The effect of time of day on the immune response after COVID-19 vaccination may be explained by the role of circadian component BMAL1 in regulating SARS-CoV-2 replication, as demonstrated by Zhuang et al. [9]. They revealed that Bmal1 silencing induced interferon-stimulated gene transcripts in Calu-3 lung epithelial cells. It provides a mechanism for the circadian pathway to limit SARS-CoV-2 infection. Here, BMAL1 is linked to COVID-19.