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PEP and Odds

May 2, 2008

Dr. Bob: I have two questions. I am on PEP. I started it about the 80th hour after potential exposure. I went to an HIV specialist who studied my symptoms and prescribed PEP.

(I am on 100 MG Norvir, 300 MG Reyataz and Truvada.) I had gone to an emergency room about the 60th hour and was prescribed the above, but only 150 MG of Reyataz. I couldnt make it any sooner as I was traveling. I then went to a specialist and was on the increased dosage of 300 MG but only after the 80th hour. The specialist encouraged me to stay on PEP.

My first question is a two parter. Where does the justification for the 72 hour PEP timeline come from. STarting in the 50th or 60th Hour, was I already too late. Also, what about the wrong dosage. If I wasnt taking enough in short enough time, is it still going to be effective? Did I miss the window so to speak?

As for my second question. Its about my 'odds'. I had receptive unprotected receptive oral sex. I had a cold sore at the time (I still do). After reading much on your website and on the internet, it looks like my odds from one time exposure would still be low--VERY LOW. But if the odds are so low for even singular receptive anal sex--50 out of 10,000, how do people get it. Its a silly question, I know, but I dont understand. I guess my question is that the odds are in my favor OR the odds are irrelevant because people still contract the virus.

Thanks for all of your help. Please send me the link to make a donation.

Response from Dr. Frascino


1. The "72 hour" limit is somewhat arbitrary, but was chosen based on what we know about the natural history of HIV infection i.e. how HIV behaves after it gains access to a new host. Once the infection becomes established, PEP would no longer be effective in aborting an infection. The general concept is that PEP has the best chance of being effective the sooner it is begun. Consequently starting PEP moments after an exposure would be better than hours, which in turn would still be better than days. Beginning PEP beyond 72 hours is generally felt not to be effective because the infection would already have been established. Please note even when PEP is started well within the 72-hour timeframe and taken exactly as directed, it is not always effective. There are PEP failures: yours truly, for example. I began PEP within minutes of my occupational exposure, but I still seroconverted. If your HIV specialist suggests PEP, even at 80 hours, I would advise you follow his suggestion.

2. Regarding your PEP dosage, you are currently taking the correct dose. I don't believe the initial low dose would affect your chances of PEP being effective. Your story of needing a dose adjustment when you consulted an HIV specialist points out graphically why I always recommend an HIV specialist be involved in the case of anyone who had an HIV exposure significant enough to warrant a course of PEP. The HIV specialist will document the HIV risk, optimize or adjust the PEP regimen (as in your case), evaluate and manage all PEP-related side effects/toxicities and finally arrange for and interpret post-PEP HIV testing.

Regarding your question on "odds," I would agree hypothetically the risk would be very low. However, there are many variables I cannot evaluate having only limited information. Significant questions that remain include: (1) how likely was it that your partner was HIV positive? (2) How severe is your cold sore? (3) How likely was it that ejaculate came into contact with the cold sore lesion? etc.

Regarding estimated HIV-risk statistics, these can be confusing. Estimated-statistical risk based on large-scale population studies can not be applied as an actual risk statistic for one specific sexual coupling, because there are far too many confounding variables that come into play. I'll reprint some information below from the archives that discusses these estimated HIV-risk statistics that will hopefully help to clarify this point.

Finally, I should perhaps also comment on Reyataz dosing, as this can be a point of confusion. (It certainly was for your ER doctor who initially prescribed your PEP regimen.) The FDA-approved dose of Reyataz is 400 mg (two 200-mg capsules) taken once per day. However, a more frequently prescribed dose is one 300-mg capsule (or two 150-mg capsules) of Reyataz plus a single 100-mg Norvir (ritonavir) capsule, both taken once per day. Norvir is used in this situation to boost Reyataz levels in the bloodstream, thereby making it more effective against HIV. It's also important to note this combination of Reyataz boosted by Norvir is necessary for HIVers who are also using either Viread (tenofovir) or Sustiva (efavirenz). This is because both Viread and Sustiva can decrease Reyataz levels in the bloodstream.

Good luck with your "PEPing."

Dr. Bob



Ive written to you many times over the past 3 years and youve answered several of my questions. Thank you! What I really need to know now is how accurate are your statistics about oral and anal sex. Is it really 1 per 10,000 for oral and 50 per 10,000 for anal? Im trying to explain to my negative partner exactly what our specific risk is. Neither one of us are math whiz-kids but this seems reasonably straight forward. He could expect to become infected once for every 10,000 blowjobs. Right?

Thanks Dr. Bob

Response from Dr. Frascino


"He could expect to become infected once for every 10,000 blowjobs. Right?"??? Well actually no, that would be a wrong conclusion to draw from those statistics!!! I've covered this topic numerous times in the past, but I know HIV statistics can be a confusing topic. So even though this questions has now become a QTND (question that never dies) with an ATNC (Answer that never changes), I'll try to explain the limitations of these statistics once again.

The statistics I quoted are "estimated per-act risk statistics for acquisition of HIV by various exposure routes" published in a CDC document. These statistics were generated by combining a variety of published reports and did not control for many different potential variables that occur in different populations and among individuals. In other words, these statistics are primarily useful in determining relative risk, but not specific risk or actual risk for any individual. The reason for this is that any specific sexual coupling has a wide variety of variables to take into consideration when attempting to quantify specific HIV-transmission risk. These would include both viral factors, such as viral strain and viral load, as well as host factors, such as immune integrity, concurrent illnesses, circumcised/uncircumcised, genetic susceptibility, etc. Add to this nonspecific factors/extenuating circumstances, such as roughness of the encounter possibly causing trauma to mucous membranes, menstruation, etc., and perhaps you will begin to see the difficulty in providing transmission-risk statistics for any specific coupling. Also I should point out we cannot conduct prospective controlled epidemiological studies to try to account for theses variables, as that would be unethical. There are some published reports that address risk associated with specific sexual practices that control for some variables, but these studies usually have relatively small sample sizes and again are not applicable to everyone's specific situation. Another reference that I quote frequently is (SAFER SEX METHODS). If you review the specific epidemiologic studies in this well referenced report, you'll get a better understanding of the complexity involved in these issues.

So why do I quote the statistics that I do? Good question! The main reason is that I am constantly barraged by anxious wrecks desperately trying to quantify their risk. I use the CDC statistics, because CDC is a very conservative organization and the numbers they generated are an amalgamation of many studies. They also standardized the relative risk to a common denominator ("10,000 exposures to an infected source"), which allows us to discuss relative risk. For instance, unprotected receptive anal sex is approximately 10 times more risky than unprotected insertive penile-vaginal sex, which in turn is approximately 10 times more risky than unprotected insertive oral sex.

I hope that this will help clarify the limitations of these estimated HIV-transmission risk statistics.

The bottom line is really much more concrete and easy to comprehend. If someone has placed himself or herself at risk for HIV, he or she should be HIV tested. Period. End of story.

I can just about hear all the paranoid panicky worried wells beginning to type away furiously, providing me with a blow-by-blow of their latest blow-by-blow and begging for me to quantify their specific risk. But unfortunately, unless the other person they were having sex with was me, I will not have enough specific detail to give them an accurate response. Hell, even if it were me, I still might not be able to give a completely accurate risk quantification!

Dr. Bob


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