15 During viral infection, the numbers of virions infecting individual cells can vary significantly over 16 time and space. The functional consequences of this variation in cellular multiplicity of infection 49 Cellular co-infection plays an important, yet poorly defined role in shaping the outcome of 50 influenza A virus (IAV) infection. By facilitating reassortment between incoming viral genomes, 51 cellular co-infection can give rise to new viral genotypes with increased fitness or emergence 52 potential (1). Cellular co-infection can also enhance the replicative potential of the virus by 53promoting the complementation and multiplicity reactivation of the semi-infectious particles that 54 constitute the bulk of influenza virus populations (2-4). Despite its clear importance, the 55 prevalence and the specific functional consequences of cellular co-infection during IAV infection 56 remain largely unknown. 57 58We and others have previously shown that cellular co-infection can be common in vivo (5,6). 59There is growing evidence that IAV replication and spread is focal and thus that the distribution 60 of individual virions across cells and tissues is highly spatially structured, resulting in foci of high 61 cellular multiplicity of infection (MOI) (7-10). Given the dynamic distribution of virions over time 62and space during infection, it appears likely that the MOIs of individual infected cells can vary 63 significantly. This raises the question whether this variation in the number of virions that infect a 64 cell has distinct phenotypic consequences. If so, it could have significant implications for 65understanding IAV infection dynamics as two viral populations of identical size and genome 66 sequence could give rise to divergent infection outcomes if the dispersal patterns of virions (and 67 thus the MOI distribution across cells) differs. 68 69Several previous studies have suggested that the number of virions that enter a given cell 70 (referred to throughout as "cellular MOI" or "viral input") may affect replication kinetics and 71 interferon (IFN) induction (11-14); however, the phenotypic consequences of cellular MOI during 72 IAV infection have not yet been rigorously or comprehensively quantified. Here, we combine 73 precise single-cycle infection experiments with statistical model fitting to reveal that cellular MOI 74 significantly alters virus production rates, the host response to infection, and the potential for 75 further superinfection. In doing so, we precisely define the functional forms that govern the 76 relationships between the amount of viral input and the phenotypes of infected cells, information 77 that will aid future efforts to quantitatively model IAV infection. Altogether, these results reveal 78and define an unappreciated role for cellular co-infection in shaping the outcome of IAV infection. 79 80 RESULTS 81To define how variation in cellular MOI affects viral replication dynamics and the host response 82to infection, we infected either MDCK or A549 cells with A/Puerto Rico/8...