CD4 and infection: Cellular immune responses can be broad but vary widely (Grifoni et al., 2020; Peng et al., 2020; Sekine et al., 2020; Rydyznski Moderbacher et al., 2020; Braun et al., 2020; Zhou et al., 2020), and lymphopenia is a prominent feature of more severe infection, affecting CD4+ and CD8+ T cells, as well as B cells (Tavakolpour et al., 2020; Chen and John Wherry, 2020; Huang et al., 2020; Giamarellos-Bourboulis et al., 2020; Tan et al., 2020; Kuri-Cervantes et al., 2020; Mathew et al., 2020).