"Always"	100%
	"Most of the time"	80%
	"About half the time"	50%
	"Rarely"	20%
	"Never"	0%

then by summing the percentages for all of the respondents engaging in the risky behavior, and finally by dividing this sum by the number of respondents. For example, 52 of the 180 respondents admitted to "using a needle that someone else had used before you" during the past three months. Among these 52, the reported frequency of bleach was "always" for 18, "most of the time" for 8, "about half the time" for 16, "rarely" for 5, and "never" for 5. The Overall Prevention Frequency is then given by (18 x 1.00) + (8 x .80) + (16 x .50) + (5 x .20) + (5 x .00), divided by 52. The result is .64, which means that there is roughly a 64% usage of appropriate prevention by the clientele.

Another way to look at the data is to calculate "Per Capita Risky Acts". Respondents who reported injecting drugs were asked "how many times you used a needle that someone else had used before you". By combining, for each individual, the number of needle-sharing acts times the frequency of bleaching, that person's approximate number of risky acts can be calculated. The overall sum of risky acts for 67 injectors (including 19 who reported zero needle-sharing) was 547. That averages out to 8.2 Per Capita Risky Acts in three months, or about 33 PCRA per year.

We can now attempt an estimate of the seroconversion rate. Let's use the Kaplan and Heimer estimate that the probability of HIV transmission in one needle-sharing act is .0067. And let's use the Consensus Report estimate that 16% of IDUs (straight + gay) in San Francisco are HIV-infected. We can then estimate the per-capita annual risk of seroconversion as

(Per Capita Risky Acts) x (Per-act transmission probability) x (Proportion of all acts done with HIV+ partners)

= 33 x .0067 x .16 = .035

Put another way, for every 100 person-years in a population behaving like our HAFCI sample, there will be 3.5 seroconversions- an unacceptably high rate, given that we hope that IDU seroconversions can be kept below 1.0 per 100 person-years. A close look at the data, however, reveals that most of the risk is borne by a small minority: 8 of the 67 accounted for nearly three-quarters of all the risky acts.

Tops and Bottoms and HIV Risk

There were 78 males in the HAFCI's Behavioral Risk Assessment sample. Of these, 51 reported male-to-male (MSM) sexual activity during the past three months. They had four kinds of risk, which in descending order of severity were (A) being a "bottom" or receptive partner in anal intercourse, (B) being a "top" or insertive partner in anal intercourse, (C) being a fellator, or one who takes a penis into his mouth, and (D) being a fellatee, or a male who is stimulated orally. The 51 homosexually active men distributed as follows:

	Bottom predominantly	11
	Top-and-bottom	21
	Top only	10
	Fellator-and-fellatee	7
	Fellatee only	2

Of the 32 who reported anal-receptive activity, 16 said that condoms were used "always" or "most of the time"; the other 16 reported condom use "about half the time", "rarely", or "never". The Overall Prevention Frequency was 58%. Collectively, the 32 men engaged in about 415 high-risk acts (receptive anal intercourse without a condom) in the three months prior to their BRA interviews. That's a rate of 1,660 risky acts per year, or 33 per capita among the homosexually active men. Again by using the equation

Per Capita Seroconversion Risk

=

(Per Capita Risky Acts) x (Per-act transmission probability) x (Proportion of all acts done with HIV+ partners)

and estimating the per-act transmission risk at .0025 and the proportion of sexual partners who are HIV+ at 30%, we get

   33 x .0025 x .30 = .025

as the annual seroconversion risk for homosexually active men from receptive anal intercourse. As with the IDUs, most of the risk is borne by a small minority: just 7 men (notably, all seven were versatile top-and-bottoms) accounted for 86% of all of the risky acts.

The MSMs are also at risk from unprotected insertive anal sex. The respondent group reported a total of 264 such risky acts in the past three months, or 21 per capita per year. Their Overall Prevention Frequency was 66%. Assuming a lower per-act seroconversion risk- my educated guess is that insertive is about one-tenth as risky as receptive anal sex- it is seen that the "top" activities only adds a small amount to the MSMs' overall risk. Interestingly, among the 10 men who were exclusive tops during the past three months, eight reported "always" using condoms for anal sex, while one reported such use "about half the time" and one "never". For them, the per capita rate of risky acts was only about 3 per year.

The upshot of all this is that the IDUs and the MSMs each average about 33 highly risk acts per year. Sharing needles may be a somewhat more dangerous act, but it is done among an IDU population whose HIV rate is much lower than that for MSMs. So the overall seroconversion risk for both groups is on the order of 3 per 100 person-years. That's an unnervingly high rate- but we should keep in mind that the estimates are biased by those few respondents who report huge numbers of risky acts, and anyway the HAFCI respondent group may not be representative of the behavior of San Francisco's IDUs and MSMs as a whole.

Safer Oral Sex: Don't Waste Your Breath

In the BRA interviews, people were asked about prevention measures in oral sexuality. For those giving or receiving cunnilingus, the question was phrased, "how often was a barrier like saran wrap or a dental dam used- always, most of the time, about half the time, rarely, or never?" Among the 84 reportages of cunnilingus prevention measures, and the 150 reportages of fellatio prevention measures, these were the results:

		Cunnilingus	Fellatio
	"Always"	0	5
	"Most of the time"	0	8
	"About half the time"	2	5
	"Rarely"	4	19
	"Never"	78	113

Let's assume the respondents are telling the truth (if anything, they're likely to overstate their prevention measures to please the interviewer) and then calculate an Overall Prevention Frequency as above. The result is .02 for cunnilingus and .12 for fellatio- that is, prevention measures are claimed for only about 2% of instances of cunnilingus and 12% of fellatio. There was no significant correlation of barrier use with frequency of acts for either cunnilingus or fellatio- i.e., people who did it a lot weren't more likely to seek protection.

Leaving aside the question of whether there is a significant HIV risk for oral sex, it is obvious that this respondent group is not buying the "use a barrier!" message- and this in a city which has had perhaps the heaviest barrage of prevention education of any city in the world. Even the gay men in this group had only a 5% Overall Prevention Frequency for fellatio. Of course, clients at HAFCI Detox and HAFCI Womens Needs Center might be less compliant with the City's prevention urgings- but hey, they're representative of just the kind of high-risk people we're supposed to reach!

If these findings are confirmed by other BRA sample groups, we should seriously consider not wasting our breath anymore trying to get San Franciscans to "practice safer oral sex". And we should stop asking about oral sex in these interviews, but focus only on the real risks.

(MidCity Numbers, July 1998)