MOI (Multiplicity of Infection) Calculators

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Multiplicity of infection (MOI) is the ratio of infectious agents (viruses, bacteria, phage, or viral vectors) to target cells (or bacteria) in an infection or transduction experiment. MOI = number of infectious particles / number of target cells. At MOI = 1, there is on average one infectious particle per cell; at MOI = 10, ten particles per cell on average. Because the actual number of particles infecting any individual cell follows a Poisson distribution, even at MOI = 1 approximately 37% of cells will be uninfected. Understanding MOI is essential for designing virology experiments, gene delivery protocols, and phage therapy.

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MOI Formula

MOI = number of infectious particles / number of target cells

For viruses: particles may be expressed as PFU (plaque-forming units), TCID₅₀ (50% tissue culture infectious dose), or TU (transducing units for lentiviral vectors).

Example: infect 1 × 10⁵ cells with 5 × 10⁵ PFU: MOI = 5 × 10⁵ / 1 × 10⁵ = 5.0.

Poisson Statistics and MOI

The probability that a cell receives exactly k particles follows the Poisson distribution: P(k) = e^(−MOI) × MOI^k / k!. Key outcomes: Fraction uninfected = e^(−MOI). At MOI = 1: P(0) = e^(−1) = 0.368 (37% uninfected); P(1) = 0.368; P(≥1) = 0.632 (63% infected). At MOI = 3: P(0) = e^(−3) = 0.050 (5% uninfected); P(≥1) = 95% infected.

Choosing MOI for Experiments

  • MOI = 1–5: for measuring viral replication kinetics; ensures most cells are infected while limiting multiple infections
  • MOI = 10–100: for gene delivery/transduction to ensure high transduction efficiency (>90%)
  • MOI = 0.01–0.1: for studying viral spread dynamics (most cells initially uninfected)

MOI for Lentiviral Transduction

Typically MOI = 5–10 for 70–90% transduction of primary cells; MOI = 50–100 for difficult-to-transduce primary cells (hematopoietic stem cells). Too high MOI → multiple integrations per cell → genotoxicity risk; too low → insufficient transduction. Titrate viral vector on your specific cell type and target locus before large-scale use.

Glossary

MOI (Multiplicity of Infection)
Ratio of infectious particles to target cells: MOI = particles/cells; actual infections follow a Poisson distribution; MOI = 3 gives ~95% of cells infected; used in virology and gene delivery.
Poisson Distribution (MOI)
Fraction of cells with k infections: P(k) = e^(−MOI) × MOI^k / k!; fraction uninfected = e^(−MOI); used to calculate expected infection efficiency at any given MOI.
PFU (Plaque-Forming Unit)
A measure of viral infectivity: the number of viral particles capable of forming a plaque (clear area of cell death) on a monolayer; used to calculate MOI in cell infection experiments.

Frequently Asked Questions

MOI (multiplicity of infection) = infectious particles / target cells. If you have 1 × 10⁶ cells and add 5 × 10⁶ viral particles: MOI = 5 × 10⁶ / 1 × 10⁶ = 5. MOI tells you the average number of infectious agents per target cell. Because actual infections follow a Poisson distribution (random), even at MOI = 1, only ~63% of cells receive at least one particle. Use the Poisson formula to calculate the expected fraction of cells with any given number of infections: P(k) = e^(−MOI) × MOI^k / k!.

P(k) = e^(−MOI) × MOI^k / k!, where k = number of particles infecting a cell. Critical values: MOI = 1: 37% uninfected (P(0) = e⁻¹ = 0.368), 37% infected by exactly 1 particle, 26% infected by ≥2 particles. MOI = 3: 5% uninfected, 15% with 1 particle, 22% with 2 particles, 22% with 3 particles. MOI = 10: 0.005% uninfected (essentially all infected). Choose MOI based on desired infection coverage: for high-efficiency experiments (>95% infection), use MOI ≥ 3; for single-infection kinetics experiments, use MOI ≈ 0.01–0.1.

For standard mammalian cell lines (HEK293, Jurkat): MOI = 5–10 typically gives 70–90% transduction. For primary T cells and NK cells: MOI = 5–20. For hematopoietic stem cells (HSCs): MOI = 20–100 may be needed (less efficient). For neurons: MOI = 1–5 (highly transducible with VSV-G pseudotyped lentivirus). Titrate by transducing cells with serial dilutions of the vector batch, measuring transduction efficiency (GFP+% or target gene expression) after 3–5 days. Calculate MOI from the titer (TU/mL) and cell count on the day of transduction. Avoid MOI > 20–50 for primary cells to minimize genotoxicity from multiple integrations.

MOI = (viral titer × volume) / cell number. Accurate titer is critical — using a titer that doesn't reflect infectious units in your cell type will give an incorrect MOI. Common titer units: PFU (physical infection assay); TCID₅₀ (50% infection end-point dilution); TU or IFU (transducing units measured on specific target cells). Important: titer is cell-type dependent — a lentivirus at 1×10⁸ TU/mL on HEK293 cells may have very different functional titer on primary T cells. Always verify titer on your specific target cell type. If using titer from a different cell type, your actual MOI may be significantly different from calculated.