how can the IC50 be used to determine the sensitivities of viruses to drugs

The IC50 of a drug is a measure of its ability to kill something
(typically a cell).  It is the concentration of the substance that
kills half of the targets.  Roughly speaking, the answer to your
question is "the lower the IC50, the more sensitive the virus is to
the drug".  BUT... IC50 values are really of limited value (IMHO)
since they are determined under laboratory conditions in cultures. 
Furthermore, it is by no means certain that concentration of a drug is
the only, or indeed most important contributor to efficacy.  More
important may be the ratio of drug molecules to the number of total
molecular targets present.  IC50 does not take tis into account. 
Neither does it define the time it takes for the killing to take
place.  IC50s are very often quite different for any drug/subject
combination depending on WHEN the effect is measured.

As an example, one molecule of ricin can kill one cell.  Given
sufficient time, one per cell is all you need to wipe out a culture. 
Note that this is independent on the volume (and thus the
concentration), and that the degree of killing will go up as each
molecule finds a cell and kills it.

IC50 is an abstract measure really only suitable for the evaluation of
different drugs under the same experimental conditions.

Strictly speaking from a pharmacological basis, an IC50 is the
“inhibitory concentration” needed to have a 50 percent effect on a
measurable response; in this case, this would be the growth of the
virus, and the term would more correctly be an LD50, the “lethal dose”
for the death of 50 percent of the population (be it virus, cell,
animal, etc).

In terms of having limited value, bear in mind that the LD50 measure
has been used by drug companies for tens of years as a measure of drug
toxicity (in animals, extrapolated to man), and if things haven’t
changed more recently, is an essential item of any drug regulation
application.

So here your IC50 is really an LD50, and as commented already by
xarqi.ga it will provide an indication of the sensitivity of the virus
to a specific drug, and allow direct comparisons across different
drugs when tested under the same conditions; again, an essential
component of drug screening. The lower the IC50 (in terms of a
concentration of a drug that kills 50 percent of your viruses), the
more potent the drug.

In pharmacological terms, a true IC50 can have great value. If you
have any sort of response that you can measure (like your drug killing
the viruses here) that you can INHIBIT with another drug, the
concentration of this second drug that produces a 50 percent
inhibition of the measurable response to the first drug (its IC50) can
tell you a lot. From this (knowing the concentration of the first drug
and its LD50) you can calculate a better “affinity” (a sort of
baseline potency that can be directly compared across systems) value
of your second drug. Thus, these values can then be compared with
other drugs in the same system, and you can extrapolate your system to
compare things with other systems. As this is a measure of the
inhibition of another response, it cannot be measured directly without
having the activation response there first, and hence the use of the
IC50 is essential for the understanding of the potency of the second
drug. This way of handling things comes more directly from enzyme
kinetics and drug-receptor binding interactions, which can provide
essential data for the understanding of many drug effects with their
interactions with enzymes and receptors… the basis for the whole
concept of drug design, and the millions of dollars spent every year
by drug companies, which starts with a screening for IC50s of any
possible drugs on whatever system they are interested in.