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Background:
We view HIV
as a 3-stage model in which a person progresses through the primary,
asymptomatic and symptomatic stage. A person’s infectivity, or the
probability a contact between an infected and susceptible will transmits the
disease, varies with stage of infection. We model the San Francisco “gay
epidemic.” Beginning in 1978, blood samples from 6875 men were taken and
behavioral data recorded as part of a Hepatitis-B vaccine trial. Subsequent
reanalysis of some of those blood samples for HIV provides the most accurate
incidence data describing the onset of HIV in any population. From the
behavioral data collected we determine that the SF gay population can be
broken into six sexual activity groups ranging from 231 partners per year to
none.
Objective:
To determine
how infectious HIV is.
Methods:
We use mathematical modeling which reflects the great variation in contact
rates between gay men.
Results:
T
he infectivities for the primary, asymptomatic and symptomatic stages are
0.015, 0.006, 0.223 respectively. The third stage infectivity is
significantly higher than the other two stages and ultimately drives the HIV
epidemic.
Conclusions:
A reduction of the effective contact rate (infectivity times frequency of
contacts) by a factor of 100 would have been necessary for the gay epidemic
to have ceased to be endemic. If we lower the effective contact rate by a
factor of 10, similar to Sub-Saharan Africa, large outbreaks occur but are
delayed by many years. The model suggests countries such as India have an
epidemic doubling every year.
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