Database Reference
In-Depth Information
Chapter 8
Chicken Pox and Shingles
Chicken pox, a microparasitic infection caused by the
varicella zoster
virus, is an
example of a highly infectious childhood disease. This virus can be spread either
through direct contact with infected individuals or through the air. After exposure to
the virus, the incubation period before an individual becomes contagious is approx-
imately seven days. Individuals are contagious for about seven days during which
symptoms including fever and blisters appear, and then remain sick for an additional
fourteen days. Once an individual recovers, he or she develops a natural immunity
and is unlikely to get the disease again.
However, the virus remains in the body and later in life, it manifests itself in the
form of shingles in about 15 percent of the population that contracted chicken pox.
Shingles has symptoms that are similar to chicken pox but strikes mostly individuals
over the age of 50 that are fatigued or under stress. It takes approximately 10 days
to recover from shingles and during this time susceptible individuals can contract
chicken pox from those suffering from shingles.
New vaccines are becoming available to immunize people against chicken pox.
The target population for immunization typically is children, because they comprise
the highest infectious class. Immunizing children would help to reduce occurrence
of chicken pox. However, no vaccination program can reliably cover 100 percent
of a population. Consequently, as childhood vaccination takes place, fewer cases of
chicken pox among children occur but the age of first infection increases. Since
older individuals have more difficulty dealing with chicken pox than young individ-
uals, vaccination programs may shift the burden from the young to the old.
The following model addresses several interrelated questions: How are different
population age cohorts affected by chicken pox in the absence of immunization and
shingles? What are the effects of immunization on the average age of first contraction
of chicken pox? What are the effects of shingles on the average age of first contraction?
To answer these questions, we first explore the effects of chicken pox on different
age cohorts in a given population. We then investigate what happens to the same
population when the effects of shingles are factored into the model. Finally, we
turn to the effects of childhood immunization on the average age at which a person
contracts the disease.
