Multiplicity of infection

# Multiplicity of infection

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Encyclopedia
The multiplicity of infection or MOI is the ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...

of infectious
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

agents (e.g. phage or virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

) to infection targets (e.g. cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

). For example, when referring to a group of cells inoculated with infectious virus particles, the multiplicity of infection or MOI is the ratio of the number of infectious virus particles to the number of target cells present in a defined space.

## Interpretation

The actual number of phages or viruses that will enter any given cell is a statistical
Statistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....

process: some cells may absorb more than one virus particle while others may not absorb any. The probability
Probability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...

that a cell will absorb virus particles when inoculated with an MOI of can be calculated for a given population using a Poisson distribution
Poisson distribution
In probability theory and statistics, the Poisson distribution is a discrete probability distribution that expresses the probability of a given number of events occurring in a fixed interval of time and/or space if these events occur with a known average rate and independently of the time since...

.

where is the multiplicity of infection or MOI, is the number of infectious agents that enter the infection target, and is the probability that an infection target (a cell) will get infected by infectious agents.

In fact the infectivity of the virus in question will alter this relationship. One way around this is to use a functional definition of infectious particles rather than a strict count, such as a plaque forming unit
Plaque forming unit
A plaque-forming unit is a measure of the number of particles capable of forming plaques per unit volume, such as virus particles. It is a functional measurement rather than a measurement of the absolute quantity of particles: viral particles that are defective or which fail to infect their...

.

For example, when an MOI of 1 (1 viral particle per cell) is used to infect a population of cells, the probability that a cell will not get infected is , and the probability that it be infected by a single particle is , by two particles is , by three particles is , and so on.

The average percentage of cells that will become infected as a result of inoculation with a given MOI can be obtained by realizing that it is simply . Hence, the average fraction of cells that will become infected following an inoculation with an MOI of is given by:

which is approximately equal to for small values of .

### MOI Examples

As the MOI increases, the percentages of cells infected with at least one viral particle also increases.
MOI % Infected
1.0 63.2%
2.0 86.5%
3.0 95.0%
4.0 98.2%
5.0 99.3%
6.0 99.8%
7.0 99.9%
8.0 100.0%

Fields' virology, Part 1 By Bernard N. Fields,David Mahan Knipe,Peter M. Howley,Diane E. Griffin