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The extent and origin of variation in the replication dynamics of complex DNA viruses is not well-defined. Here, we investigate the vaccinia virus (VACV) infection cycle at the single-cell level, quantifying the temporal dynamics of early and post(dna)-replicative phase gene expression across thousands of infections. We found that viral factors determine the initiation time of these phases, and this is influenced by the multiplicity of infection (MOI). In contrast, virus production dynamics are largely constrained by the host cell. Additionally, between-cell variability in infection start time and virus production rate were strongly influenced by MOI, providing evidence for cooperativity between infecting virions. Blocking programmed cell death by pan-caspase inhibition increased infection frequency but not virus production at the population level due to a concurrent attenuation of per-cell virus yield, suggesting a dual role for caspase signaling in VACV infection. Our findings provide key insights into the pivotal factors influencing heterogeneity in the infection cycle of a large DNA virus at the single-cell level. Author summary: The successful transmission and progression of a viral infection depends on the initial infectious virus dose, which in turn is defined by the efficiency of virus replication within host cells. Understanding what determines the productivity of individual cells, and the variability between them, is a central question in the emerging field of single-cell virology. Our study investigates the single-cell infection dynamics of vaccinia virus (VACV), a large, complex, DNA virus, revealing a profound influence of initial infectious dose on the timing and rate of progression of virus replication. Cells infected with a low dose take over three times longer to initiate virus production compared to those with a higher dose. However, the dynamics of the second phase of infection, de novo production of virus, are unperturbed by this variable: a higher infectious dose does not equate to a greater yield. Furthermore, we observed that the proportion of cells advancing to productive replication increases markedly with the virus dose. This effect is compounded by the suppression of apoptosis, a cell death process, suggesting that induction of apoptosis is effective in pruning the population of susceptible host cells. This has broad implications for the pathogenesis of viral diseases as the infectious dose will govern the window of opportunity for the host immune system to effectively respond to the infection. This research contributes to the broader field of single-cell virology by highlighting the complex relationship between virus dose and infection dynamics at the cellular level. [ABSTRACT FROM AUTHOR]