THE SEEDING OF THE AIDS EPIDEMIC
My previous post, "The Odd Epidemiology of AIDS" showed
how the notion of "Patient Zero" is not a likely scenario.
I.e, the AIDS epidemic appears much more likely to have
begun with multiple, simultaneous infections, rather than
a single infected person. In order to defend the "Patient
Zero" scenario, it would be necessarily to imagine that
currently quoted rates of infectivity for various sexual
acts are significantly in error.
One of the most obvious questions to ask would be:
How large of a 'seed' would be required, to account
for the rate of HIV growth observed early in the epidemic?
This is another question that can be explored with the
computer modeling program that I described, previously.
I am again examining the city of San Francisco. I previously
cited a study [1] that gave an estimate of the SF gay male
population size, as 56000. This same study estimates that
the entire SF gay male population had a level of near zero HIV
infections in 1978, rising to 49.3% HIV+ in 1987 (a span of
9 years). The first AIDS case in SF was diagnosed in 1981.
To be conservative, I'll push the starting date back to 1976
as the first year in which HIV infection occurred (a span
of 11 years until 1987)
We are looking for an "initial infections" figure
that leads to (56000 * .493 = 27608) infections
in 11 years. This would be the number of men who would have
all had to become HIV+ in the starting year, in order to account
for the subsequent growth pattern.
In order to estimate this, I ran the program repeatedly,
trying different initial values, starting with 200 men,
and working up.
I am estimating 67 contacts per year, a figure previously cited
as representative of SF gay males appearing at the city's
main VD clinic. I am estimating an even mix of anal and
oral sex, with infection probabilities of .0082 and .0006,
respectively, for the high-risk receptive roles.
The bottom line: it appears that it would have taken
between 2000 and 2200 men to become mass-infected, in one
year (excerpts of program outputs, appended below).
How did that happen? Perhaps 2000 men vacationed in Haiti
and picked up a disease? Perhaps they made a carvan to
Africa and drank too much monkey blood, or were bitten
by chimps?
Even if 2000 men traveled to Haiti, and each had a half-dozen
anal receptive encounters with HIV+ partners, only about
49 would have become infected. The problem with getting 2000
simultaneous infections in the starting year, is that you
need far in excess of 2000 sex acts with HIV+ partners,
in order to infect those 2000 men. From where did the HIV+
partners come, to infect that many men, all at once?
On the other hand, if you have a hepatitis study starting
in 1978, involving thousands of gay men, trusting their lives
to a government infested with hate-mongers, that has a long and
sordid history of covert assassination/torture/experimentation...
That which increaseth knowledge will also increaseth sorrow.
Tom Keske
Boston, Mass
[1] Lemp, et al, "Projections of AIDS Morbidity and Mortality
in San Francisco. PMID: 2407871 UID: 90172481
-----------------------------------------------------------
* TRYING INITIAL INFECTION SIZE = 200
Enter Population Size ( <= 100000): 56000
Enter number initially infected: 200
Probability, infection per ACTIVE contact, type #1: .0082
Probability, infection per PASSIVE contact, type #1: .0082
Probability, infection per ACTIVE contact, type #2: .0006
Probability, infection per PASSIVE contact, type #2: .0006
Enter average number of contacts per year: 67
Enter number of years: 11
New infections in year #11 = 1068, GRAND TOTAL = 4686
-----------------------------------------------------------
* TRYING INITIAL INFECTION SIZE = 1000 (all other parameters
are unchanged)
Enter number initially infected: 1000
New infections in year #11 = 3445, GRAND TOTAL = 17754
-----------------------------------------------------------
* TRYING INITIAL INFECTION SIZE = 1200
New infections in year #11 = 3575, GRAND TOTAL = 19850
-----------------------------------------------------------
* TRYING INITIAL INFECTION SIZE = 1500
New infections in year #11 = 3888, GRAND TOTAL = 23238
-----------------------------------------------------------
* TRYING INITIAL INFECTION SIZE = 1600
New infections in year #11 = 3944, GRAND TOTAL = 24695
-----------------------------------------------------------
* TRYING INITIAL INFECTION SIZE = 1700
New infections in year #11 = 3978, GRAND TOTAL = 24868
-----------------------------------------------------------
* TRYING INITIAL INFECTION SIZE = 2000
New infections in year #11 = 4103, GRAND TOTAL = 26557
-----------------------------------------------------------
* TRYING INITIAL INFECTION SIZE = 2200
New infections in year #11 = 4207, GRAND TOTAL = 28469
----------------------------------------------------------
