Another report demonstrated that co-infection caused rapid infiltrations of monocytes, B cells, CD4+ T cells, and CD8+ T cells in the bronchoalveolar lavage fluid in infected mice and led to severe lymphopenia in peripheral blood, which is strongly associated with the impaired production of IgG, and neutralizing antibodies against SARS-CoV-2 and the influenza virus, which increased disease severity [31]. Here, CD4 is linked to coinfection.