Hi, and thanks for your question
Condoms, when used properly, are a very effective barrier against HIV transmission. They are also excellent at preventing other sexually transmitted infections such as chlamydia, gonorrhea, and syphilis as well as preventing pregnancy.
For maximum efficacy it’s important that condoms be used correctly. A new condom should be used every time you have intercourse, as well as with each new partner, and when switching from anal to oral or vaginal penetration. The condom should be put on before the penis/external genitals touch the partner’s genitals or anus.
I don’t have any data on whether there has ever been a case of HIV with an intact condom, but what I can say is that condoms are very effective at preventing HIV.
Have a look at our Condoms page for more information and tips. You might also find our Know Your Chances charts helpful when considering risk of various types of STIs including HIV.
Let us know if this does not answer your question or if you have any more questions or concerns.