We describe development of mRNA-based tests to quantify activity of the JAK-STAT1/2 and JAK-STAT3 signal transduction pathways in immune cells, and based on an evaluation on multiple public clinical studies (influenza, RSV, yellow fever, dengue, rotavirus infections) provide evidence for value of the tests to quantify the host cellular immune response to a viral infection or vaccine. This evidence concerns the gene STAT3 and viral infectious disease.