MOI / Infection Planning Calculator

What Is Multiplicity of Infection (MOI)?

Multiplicity of infection (MOI) is the ratio of infectious agents (e.g., virus particles) to infection targets (e.g., cells). It is the single most important parameter in any transduction, infection, or gene delivery experiment.

At MOI = 1, each cell gets an average of one virus particle. However, because particles distribute randomly, the actual number per cell varies. The Poisson distribution describes this: at MOI = 1, about 36.8% of cells receive zero particles, 36.8% receive exactly one, 18.4% receive two, and 6.1% receive three.

The Poisson Model of Viral Infection

The probability that a cell receives exactly k virus particles at a given MOI follows the Poisson distribution:

This model assumes three things: (1) virus particles are uniformly mixed, (2) each particle has an equal probability of infecting any cell, and (3) particles behave independently. These assumptions hold well in suspension culture with thorough mixing.

In practice, the Poisson model slightly overestimates the uninfected fraction because it assumes ideal mixing. Cell aggregation, surface attachment, and receptor saturation can all cause deviations. Despite this, the Poisson model remains the standard for experiment planning because it provides a reliable conservative estimate.

Practical Tips for Viral Transduction

Always determine your virus titer experimentally before starting large-scale experiments. Vendor-supplied titers are measured on specific cell lines (often HEK293T) and may not reflect performance on your target cells.

Freeze-thaw cycles reduce lentivirus titer by approximately 50% per cycle. Aliquot virus stocks into single-use volumes immediately after production. Store at -80°C.

For lentiviral transduction, polybrene (hexadimethrine bromide) at 4–8 µg/mL enhances transduction by neutralizing charge repulsion between viral and cell membranes. Some cell types (especially hematopoietic cells) are sensitive to polybrene — test toxicity first or use protamine sulfate as an alternative.