At an (unrealistically?) independent 0.5% chance per act, a 50% chance of transmission would require 139 sex acts — hardly “years and years”.
I don’t see why epidemiology should care about the 50% threshold. The relevant number is the expected number of transmissions per year. Thus independence is irrelevant.[1] At 200 anal tops per year per infected person, incidence should double yearly. And every top requires a bottom, so that’s 400 anal sex acts per year for just doubling. It seemed to spread more quickly than that, but maybe 800 and 4x per year works. It seems just barely plausible with this transmission rate. I’m not sure of the details of bathhouses, but I thought that there was a lot of non-anal sex, too.
[1] independence is relevant if 70s gays were systematically different from the people in the study; and they probably were, eg, they probably had higher rates of STDs
And every top requires a bottom, so that’s 400 anal sex acts per year for just doubling. It seemed to spread more quickly than that, but maybe 800 and 4x per year works.
It took 8 years (until August 1989) for the first 100,000 cases to be reported; the second 100,000 were reported in just 2 years (by November 1991). The half million total was passed in October of 1995.
This seems to indicate a doubling time of about 2 years.
ETA: Also according to that page, the patient with the first confirmed HIV infection died of AIDS in 1968, so the growth rate of AIDS before 1989 was at most 1.73x per year.
OK, maybe doubling works. But it’s important to distinguish different populations. You should expect it to spread faster through the bathhouse scene than through the rest of the gay community than through the straight community. So it should slow down once it exhausted the bathhouse regulars (something like weekly visits) or when AIDS shut down that scene. If that happened around 1985 and there’s a 10 year incubation period, then the 1995 numbers still include bathhouse effects. Diagnoses were increasing at 3x or 4x in the early 80s: 100 in fall ’81 to 250 in mid ‘82 to 1000 in early ‘83 to 3000 by the end of the year. From ‘83 to ’89 it was merely doubling and it slowed after that. Of course, there are problems with diagnosis numbers early in an epidemic, but death followed quickly, in weird ways, so these numbers are probably good enough. Yes, there were people with AIDS in 60s, but that 1.73x includes time to get to the bathhouse scene.
One problem with this simplified model is assuming every sexual act is with a new partner, which would only be true in the very early stages.
I think your analysis is on the right track though, and it seems barely plausible with this transmission rate, assuming negligible condom use and an intense bathhouse scene. However, in standard theory HIV progresses to AIDS in about 10 years, so this sets a timer which starts removing vectors from the population.
Thus the exact exponent matters considerably. If incidence can only double every year, then after 10 years you get 2^10 ~ 1000 cases.
If incidence doubles every 6 months (quadruples every year), then you get a million cases after ten years.
If you consider that all other STD’s would infect this population before HIV, then one has to wonder how that would effect condom use, and how that changes the model.
So do you find the transmission rate and make the model before you decide that HIV was an STD which spread this way, or after?
One problem with this simplified model is assuming every sexual act is with a new partner, which would only be true in the very early stages.
It assumes that every sexual act is with an uninfected partner. Perhaps that’s what you meant, but then I wouldn’t have used “very.”
If you consider that all other STD’s would infect this population before HIV, then one has to wonder how that would effect condom use, and how that changes the model.
I think this is pretty well documented. STDs were routine and not a big deal (treatable!) in the 70s gay scene. Thus they did not cause condom use.
I don’t see why epidemiology should care about the 50% threshold. The relevant number is the expected number of transmissions per year. Thus independence is irrelevant.[1] At 200 anal tops per year per infected person, incidence should double yearly. And every top requires a bottom, so that’s 400 anal sex acts per year for just doubling. It seemed to spread more quickly than that, but maybe 800 and 4x per year works. It seems just barely plausible with this transmission rate. I’m not sure of the details of bathhouses, but I thought that there was a lot of non-anal sex, too.
[1] independence is relevant if 70s gays were systematically different from the people in the study; and they probably were, eg, they probably had higher rates of STDs
Why “more quickly than that”? From Epidemiology of HIV/AIDS in the United States:
This seems to indicate a doubling time of about 2 years.
ETA: Also according to that page, the patient with the first confirmed HIV infection died of AIDS in 1968, so the growth rate of AIDS before 1989 was at most 1.73x per year.
OK, maybe doubling works.
But it’s important to distinguish different populations. You should expect it to spread faster through the bathhouse scene than through the rest of the gay community than through the straight community. So it should slow down once it exhausted the bathhouse regulars (something like weekly visits) or when AIDS shut down that scene. If that happened around 1985 and there’s a 10 year incubation period, then the 1995 numbers still include bathhouse effects. Diagnoses were increasing at 3x or 4x in the early 80s: 100 in fall ’81 to 250 in mid ‘82 to 1000 in early ‘83 to 3000 by the end of the year. From ‘83 to ’89 it was merely doubling and it slowed after that. Of course, there are problems with diagnosis numbers early in an epidemic, but death followed quickly, in weird ways, so these numbers are probably good enough. Yes, there were people with AIDS in 60s, but that 1.73x includes time to get to the bathhouse scene.
One problem with this simplified model is assuming every sexual act is with a new partner, which would only be true in the very early stages.
I think your analysis is on the right track though, and it seems barely plausible with this transmission rate, assuming negligible condom use and an intense bathhouse scene. However, in standard theory HIV progresses to AIDS in about 10 years, so this sets a timer which starts removing vectors from the population.
Thus the exact exponent matters considerably. If incidence can only double every year, then after 10 years you get 2^10 ~ 1000 cases.
If incidence doubles every 6 months (quadruples every year), then you get a million cases after ten years.
If you consider that all other STD’s would infect this population before HIV, then one has to wonder how that would effect condom use, and how that changes the model.
So do you find the transmission rate and make the model before you decide that HIV was an STD which spread this way, or after?
It assumes that every sexual act is with an uninfected partner. Perhaps that’s what you meant, but then I wouldn’t have used “very.”
I think this is pretty well documented. STDs were routine and not a big deal (treatable!) in the 70s gay scene. Thus they did not cause condom use.