MOI, pfu, and TCID_{50}
Plaque forming units (pfu) is a measure of number of infectious virus particles.
It is determined by plaque forming assay.
Multiplicity of infection (moi) is
the average number of virus particles infecting each cell.
TCID_{50} is the tissue culture infectious dose
which will infect 50% if the cell monolayers challenged with the
defined inoculum. If the titer is "10^{3}
TCID_{50}/0.2 ml, MK, 2 days," it means that when
a 0.2 ml inoculum of a 1:1,000 dilution of the virus is added to each
of four tubes containing monkey kidney (MK) cells, two tubes are
expected to become infected.
MOI is related to pfu
by the following formula:
Multiplicity of infection (moi) = Plaque forming units (pfu) of virus used for infection / number of cells.
For example, if 2x10^{6} cells is infected by 50 ml of virus
with a titer of 10^{8} pfu/ml. The moi will be
0.05*10^{8}/2*10^{6} = 2.5.
The fraction of cells that are not infected is
P(0) = 1  e^{moi},
i. e., 8% for moi = 2.5.
To ensure 99% of cells are infected requires moi > 4.6.
Assume the conditions used for plaque assay and TCID assay don't alter the
expression of infectious virus, TCID_{50}/ml and pfu/ml are related by
pfu/ml = 0.7 * TCID_{50}.
As a working estimate, one can use
pfu/ml = 0.5 * TCID_{50}.

