What is multiplicity of infection?

MOI is the ratio of infectious viral particles to target cells in an infection. An MOI of 5 means five infectious particles per cell on average. It controls how heavily cells are infected and is a key variable in virology, transduction, and gene therapy experiments.

How is MOI calculated?

Multiply the titre in particles per mL by the volume added in mL to get the total infectious particles, then divide by the number of cells. So a 1e8 pfu/mL stock, 0.5 mL added to 500,000 cells, gives 5e7 particles over 5e5 cells, an MOI of 100.

Why use the Poisson distribution?

Particles do not divide evenly; they land on cells at random. The Poisson distribution describes that randomness, so the fraction of cells receiving at least one particle is 1 minus e to the minus MOI. This is why even at MOI 1 about a third of cells escape infection entirely.

What MOI gives near-complete infection?

Because the uninfected fraction is e to the minus MOI, you need a fairly high MOI for full coverage. MOI 3 leaves about 5 percent uninfected, MOI 5 about 0.7 percent, and MOI 10 around 0.005 percent. Choose the MOI from how complete you need the infection to be.

How do I find the volume for a target MOI?

Enter a target MOI and the tool back-calculates the volume to add. The particles needed equal the target MOI times the cell number, and the volume is that divided by the titre. This is the everyday calculation for setting up a transduction at a defined MOI.

MOI (Multiplicity of Infection) Calculator

Multiplicity of infection sets how many viral particles each cell sees, and it determines everything from transduction efficiency to cytopathic effect. This calculator gives the MOI from your titre, volume, and cell number, and uses the Poisson distribution to estimate how many cells actually get infected.

How it works

The MOI is total infectious particles divided by target cells:

particles = titre (pfu/mL) x volume (mL)
MOI       = particles / cells

Because particles distribute over cells at random, the fraction of cells that receive at least one particle follows the Poisson distribution:

fraction infected = 1 - e^(-MOI)

To plan an experiment, enter a target MOI and the tool inverts this: the volume to add is target MOI x cells / titre.

Tips and example

A 1e8 pfu/mL stock, 0.5 mL added to 500,000 cells, gives 5e7 / 5e5 = MOI 100, essentially saturating infection. The Poisson behaviour is the part people forget: at MOI 1, e^-1 is about 0.37, so roughly 37 percent of cells stay uninfected even though there is one particle per cell on average. For near-complete infection aim for MOI 5 to 10; for single-copy integration in transduction studies use a low MOI, often below 0.3, so most infected cells receive just one particle.