Variable degrees of apparent cooperativity between different viral strains may impact the interpretation of experimental results (Andreu-Moreno et al., 2020). However, if strains differ in the degree of apparent cooperativity, the assumed MOI could differ between strains substantially (Figure 6). Therefore, the difference in viral titers observed late in culture may not be due to differences in viral replication rates alone (Sonawane et al., 2024; Mostafa et al., 2025). When possible, determine the degree of apparent cooperativity (‘ n ’-value, Equation 1) for each virus strain/cell type pair being studied. Development of more detailed mathematical models of within-cell virus replication may also generate novel hypotheses leading to apparent viral cooperativity (Andreu-Moreno et al., 2020).