CD4 and severe acute respiratory syndrome: Based on data from patients with SARS in 2003–2004 (caused by SARS-CoV), and the fact that most acute viral infections result in the development of protective immunity (Sallusto et al., 2010), there is a good possibility that the level of CD4+ T cells, as important immune cells, has a strong relationship with COVID-19 convalescence (Peng et al., 2020).