When similar experiments were performed with infections by WA1, Delta and Omicron, we found elevated expression levels of not only PD1 but also cytotoxic T lymphocyte-associated protein 4 (CTLA4; which is another activation/exhaustion marker) on pulmonary T cells in all infected animals, although to a significantly lesser extent in the Omicron-infected mice (Fig. 3b,c). Here, CTLA4 is linked to infection.