[Question #12441] Statistical Odds
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7 months ago
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Good afternoon,
I appreciate the Ask the Experts forum and all the expert opinions/advice the doctors provide. I have a question on statistical odds by STD. I have seen statements on the ATE forum regarding transmission likelihood (e.g. "Even without a condom, if she were HIV infected the likelihood of transmission through genital sex is less than 1 in 2000."). Are there similar stats for each STI for both protected and unprotected encounters and if so, can you kindly share them? I am also curious about the percentage of STI cases that are asymptomatic. The percentages I have seen on asymptomatic cases are all over the place. Any information you can provide would be greatly appreciated. Happy New Year!
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H. Hunter Handsfield, MD
7 months ago
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Welcome to the forum. Thanks for your question. These are complex issues with mostly not very good data, despite the specific figures that some resources -- including this forum -- sometimes state.
There really aren't very many systematic studies on STI transmission risk. The best data are for HIV, and even that isn't great. CDC published an analysis a decade ago that reviewed the literature related to the per-exposure risk of HIV, summarizing several published studies over the previous 20-30 years. Meaning that the available data are old, not accounting for newer knowledge, the widespread use of anti-HIV drugs (generally reducing transmission risk). And even then, the data were based mostly on how HIV infected people believed they were infected, and their memories of past behaviors. They also didn't account for differences in transmission risk based on time since the infected person acquired HIV -- which is higher in the first weeks or months after infection, and lower as time passes. They also couldn't control for the presence of other factors that change transmission risk, like presence of other STIs that increase risk (like herpes due to HSV2, as the strongest influence).
Here is a link to that analysis, should you care to look: https://pubmed.ncbi.nlm.nih.gov/24809629/ Some bottom lines are roughly (and I emphasize VERY roughly: the data simply are not very precise): receptive anal sex (i.e. bottom with an HIV infected male partner), around one chance in a hundred. For the penile partner if a rectal partner has HIV, around 1 chance in 1,000. Vaginal sex for the female with an infected male partner, 1 in 1,000. Insertive vaginal sex for the male with an infected female partner, around 1 in 2,500. Fellatio (penile-oral), around one in 10,000 for the oral partner in if the penile partner has HIV, 1 in 20,000 for the penile partner if the other has HIV. (That implies receiving a BJ by infected partners once daily for 55 years before virus transmission might be likely.) All these are exceedingly rough estimates that may or may not be applicable in particuaolr situations. As a general rule, successful condom use (no mis-use, no breakage) reduces these risks by an estimated 90-99%.
Data for all other STDs are even less conclusive. For genital herpes due to HSV2, assuming the infected partner isn't having an obvious outbreak, probably around one chance per 1,000 unprotected episodes of vaginal sex; a lot higher in presence of an outbreak. These data are based on the infection rate in placebo recipients in the main research documenting the effectiveness of valacyclovir to prevent transmission. Gonorrhea and chlamydia, much higher than herpes or HIV, maybe once per every 2-5 unprotected exposures.
All these figures are based on sex with infected partners. But of course most new partners are NOT infected, so transmission frequency is correspondingly lower.
HPV must be more frequently transmitted, since half of all people apparently are infected within their first three lifetime sex partners.
Data on the proportions of infections that do not result in symptoms also are sketchy, but by general consensus among experts, this is the case for at least 90% of HPV infections; probably half of all herpes infections; half of chlamydia in males and 80-90% in females; and for gonorrhea, probably only 1-2% of urethral (penile) gonorrhea in males is asymptomatic, but much larger proportions in women and in people with rectal infection. Almost all oral gonorrhea causes no symptoms at all.
Sorry that more detailed statistics are not available, but I hope these comments start to answer your questions. Let me know if anything isn't clear.
HHH, MD
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7 months ago
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Thank you for the response. My key takeaway is the odds are clearly in favor of an individual NOT acquiring an STI from a single unprotected encounter. By using condoms consistently and correctly, the chances of developing an STI decrease immensely. That said, using condoms consistently and consistently is a no-brainer in my opinion. I suppose the one exception would be gonorrhea and chlamydia ("maybe once per every 2-5 unprotected exposures"). Also, your comment "but of course most new partners are NOT infected" got me thinking: Do we know roughly how many individuals in the United States have each STI/STD? The nature of your comment leads me to believe there is data available, but, I am guessing the data would also be unreliable as individuals are either asymptomatic or do not report their condition. What are your thoughts on this?
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H. Hunter Handsfield, MD
7 months ago
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The numbers of persons infected with gonorrhea or chlamydia nationwide at any point in time probably are in the hundreds of thousands for chlamydia and tens of thousands for gonorrhea. The more important question is the proportion of potential partners you might select who are infected at any one time. For the most part, that would be under 5% for chlamydia and under 1% for gonorrhea. These are guessimates for all infections, but at any point in time most chlamydia is asymptomatic. Most gonorrhea causes obvious symptoms, but many infected people pay no attention to symptoms and don't get treated.
In general, STI rates are very low in persons who get to know each other socially before having sex; those introduced by friends or co-workers; and higher for pick-ups, say in bars. Some sex workers obviously are at higher risk, but not all of them: many sex workers are among the safest of all potential partners, depending on the extent they know the risks and protect themselves by condom use, careful partner selection, and frequent testing. For the large majority of sexually active non-monogamous persons, there is no way to be entirely free of risk, which is why such persons are wise to get tested periodically, primarily for gonorrhea, chlamydia, syphilis and HIV.
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7 months ago
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Thank you again for your response. It is helpful. For my final question, can you provide additional context on gonorrhea and chlamydia? Specifically:
1) What are the chances of getting either STI from unprotected oral? (e.g. 1 in 2000)
2) How effective are condoms at protecting against gonorrhea and chlamydia during insertive penile sex? Thank you again for your responses.
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H. Hunter Handsfield, MD
7 months ago
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1) If the oral partner has oral gonorrhea, probably it would be transmitted about 10% of the time (one exposure in ten). On average, the possibility a female partner has oral gonorrhea is probably around one chance in a thousand. Therefore, for any single oral sex exposure with a female partner, the chance you would get gonorrhea is around one chance in 10,000. That's equivalent to doing this once daily for 27 years before you would be likely to be infected. That's WITHOUT a condom.
2) A properly used condom -- in place before any contact, and it doesn't break -- is nearly 100% effective in preventing both gonorrhea and chlamydia. (But chlamydia is zero risk from oral sex.)
That completes the two follow-up comments and replies included with each question and so ends this thread. I'm glad to hear the discussion has been helpful. Happy new year!
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