The chance of HIV being passed on even though a condom was used is really quite tiny, even if it isn't totally impossible. It's probably unrealistic to expect any method of prevention to give 100% protection or to work perfectly every time.
Laboratory tests show that under conditions of perfect use, male condoms are up to 99.5% effective. The reason why a real-life effectiveness figure could be a little lower than that is that humans are not perfect. People who intend to use condoms might not do so each and every time they have sex, or they might only put a condom on halfway through the intercourse.
And people sometimes use condoms in the wrong way. They may open the wrapper with their teeth, potentially tearing the condom. They may put a condom on before the penis is fully hard. They may use a condom that is too large or too small for the penis. The condom may slip off if the penis goes soft (it's important to withdraw the penis promptly). They may continue to use the same condom for more than 30 minutes.
If these kinds of errors are made, the condom might break or fall off, making HIV transmission possible (if one partner has HIV).
It's also worth remembering that although people usually think of "protected sex" as being sex with a condom, another form of protected sex is sex that is protected by effective HIV treatment. People who are living with HIV, taking antiretroviral medications and have an undetectable viral load (i.e. extremely low levels of the virus) are highly unlikely to pass on HIV.
More on HIV Transmission Risks at TheBody.com
To find out more about safer sex, we recommend the following articles:
In addition, our Q&A experts sometimes address questions about protected sex in our "Ask the Experts" forums. Here are some of those questions and our experts' responses:
- Safe Sex and HIV
Has there ever been a CONFIRMED case of someone using proper protection and still contracting HIV?
- real effectiveness of condoms
Is it true the CDC said the failure rate of condoms is 17% for heterosexual couples attempting to prevent pregnancy? Would the failure rate of condoms for couples attempting to prevent HIV transmission be the same?