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What do the statistics mean?

Apr 10, 2013

Dear Shannon.

I hope you are well. I have already written you in one occasion and you did give me very good insights and advice - than you for that!. However, ever since your response, I have remained with a big doubt that Id appreciate if you clarified.

My partner is HIV + and I am negative. As soon as he was diagnosed, he initiated treatment and after one year ( probably earlier ) his VL is undetectable ( yupi ! ). Since we met, we have never practiced unprotected sex and had never had an incident. I get tested every 6 months and so far so good. He only performs unprotected oral sex on me ( Im a female ). Now my question... what does it exactly mean that for someone under treatment the probability of transmission is reduced by 96%? I have tried to understand the statement from a statistical point of view, and frankly I dont get my head around it. 1. Does this mean that out of 100 unprotected encounters with someone that is HIV+ under treatment, there is only a probability of 4% to get infected? 2. or that out of 100 protected encounters with someone that is HIV+ under treatment, there is only a probability of 4% to get infected?

Basically what I'm trying to find out is whether for a discordant couple like me using condoms correct and consistently every single time ( I also make always sure that there was no leakage ), there is any chances of contracting HIV. And if there is any, why would this be?

The terror I go through every time I get tested is something that is seriously affecting me and thus I need to exactly know what my risks are regardless of all the prevention means in place.

Thanks Shannon in advance for your further clarification.

Response from Ms. Southall


I'm happy to hear that answering your questions has helped with clarity for your situation.

Using condoms is effective in the prevention of HIV transmission, as long as they are used correctly and do not break or slip off.

As to the question of what risk is reduced *to*: That's a complicated one. Here's an attempt to shed some light on it. The first passage, in quotes, was provided by James Wilton, a great advocate from the Canadian AIDS Treatment Information Exchange:

“When it comes to viral load and HIV transmission, people generally want to know two things: 1) how much can treatment reduce their risk of HIV transmission and 2) what is their risk of HIV transmission when the viral load is undetectable. These are two very different questions. The 96% risk-reduction statistic helps answer the first question and shows that treatment can drastically reduce the risk of HIV transmission under certain conditions. Unfortunately, it is not possible to translate the 96% risk-reduction statistic into an answer for the second question. A 96% reduction in risk does not mean the risk is now 4%. There are ongoing studies that are enrolling serodiscordant couples (where the HIV-positive partner is on treatment and has undetectable viral load) that do not always use condoms to get a better idea of what the risk is (when undetectable) from one act of condomless sex and over time.”

Although we know having an undetectable blood viral load can greatly reduce the risk of HIV transmission, it is unclear exactly what this risk is reduced to.

In the research conducted so far, there have been no recorded HIV transmissions among heterosexual couples where the HIV-positive partner is on treatment and their blood viral load is undetectable. However, this does not mean the risk through condomless sex is zero. All of the couples studied to date have also reported using condoms often. This makes it difficult to determine the risk of HIV transmission when no condom is used.

Although there have been no studies among gay men and other MSM, there has been one report of HIV transmission occurring between two men when the HIV-positive partner had an undetectable viral load.

Also, the risk of HIV transmission when the viral load is undetectable may not be the same for all types of sex. This risk may be higher for anal sex than for vaginal sex, particularly if the HIV-negative partner is the receptive partner (bottom) during anal sex. This is because receptive anal sex generally carries a higher baseline HIV risk than other types of sex.

There are ongoing studies following serodiscordant heterosexual and same-sex couples who are taking HIV treatment, have an undetectable viral load, and do not always use condoms. These studies will provide a better understanding of the risk of HIV transmission when the viral load is undetectable.

On a personal note: My husband gets tested too about every 6 months and I know that my stomach churns during this time. But the thing we know is that we are doing everything to maintain his negative status and with all the studies showing that having an undetectable viral load and condom use we know and feel comfortable that he will remain so. My wish for you is to get to a point of not having the "terror" feeling when it's time to be tested because you know that you are doing everything to protect yourself and loving your partner.

Be well and stay safe,


Editor's note: This response was edited on May 23, 2013, to provide a deeper look at research in this area, and to further address the complexities of the original question. You can also read another response to this question. Thanks to all who reached out with feedback!

please answer my ?
Question about receiving unprotected oral sex and flu like symptoms within 48 hours

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