Since condoms only cover the penis, is the risk of skin to skin STDs moderate(?), low(?), very low(?) when condoms are used correctly and without failure? And, is the STD risk of a man receiving unprotected oral sex from a woman very low? Are there statistics or odds you can provide?
Thank you for this question, and for keeping it brief and to the point. Although the question is short, this is a complex topic.
Male condoms indeed are most effective against infections transmitted by infected secretions and/or exposure of the urethra, and less effective for those infections transmitted by skin-skin contact. For those transmitted primarily through secretions -- gonorrhea, chlamydia and the other bacteria that can cause nongonococcal urethritis, trichomonas, HIV, and hepatitis B -- a properly used condom that remains intact is nearly 100% protective. On the other hand, the real-world effectiveness against these STDs averages around 90%, just as for pregnancy. This lower figure is called "use effectiveness", i.e. takes into account that consistent and proper use can be difficult, and condoms sometimes break.
For the skin-to-skin infections, i.e. herpes, syphilis, and human papillomavirus, condoms clearly are less protective even when properly used and intact, but few data are available on the quantitative difference. Here is a thumbnail of what is known and not known about each of those 3 STDs.
Genital herpes: Among consistent condom users, the use-effectiveness of condoms against genital herpes probably is around 90%, similar to the data for HIV, chlamydia, etc. But how much of the 10% failure rate is due to skin-skin contact above the condom versus failure to always use condoms correctly is not known. The safety factor in protection against herpes is that the infected person isn't always infectious -- i.e. the virus is actually present in active form and can be transmitted only a small percent of the time. This makes it very hard to get good data on condom effectiveness.
Syphilis: No data at all are available. However, there are many centuries of consistent belief by both persons who use them and by health professionals that they are highly effective against syphilis; and the large majority of infectious syphilis lesions are located in places where the condom would expect to cover. However, syphilis is rare in most populations the US and most western countries; for most sexually active people it is not a serious issue.
HPV: This is the now the best studied STD in relation to condom use, except for HIV. The data show that women who use condoms correctly and consistently for all vaginal sex with men have about a 60% lower rate of new HPV than women who don't use condoms. For women who use condoms less than 60-70% of the time, there is no difference in HPV rates compared with non-users. (There are no data for female to male transmission, but probably it is about the same.) In other words, the very best use-effectiveness probably is no better than 60%, which means that even those who always use condoms correctly still get HPV frequently. This is one of the reasons that genital HPV infection is more or less inevitable; it happens at least once to all sexually active people, and many people acquire multiple HPV infections over time.
It is logical and generally believed that the female condom, which provides greater skin coverage for both partners, probably is more effective against the skin-to-skin STDs than male condoms, but no data exist to prove or disprove this idea.
Does any of this mean that non-monogamous sexually active persons should not use condoms? Of course not. We do not demand 100% effectiveness in order to recommend any safety device. Seat belts and smoke alarms are advised even though some people die despite using them.
I hope this helps, and that others will find the information useful. Thanks again for bringing it up.
I forgot to reply to the question about oral sex. Oral sex and STD/HIV riskhas been discussed innumerable times on this and the HIV Prevention forum. Many discussions are available. In general, oral sex is very safe for all STD. Condoms reduce the already tiny risk to zero, probably for all STDs, but most people find condoms for oral sex distasteful, but literally and figuratively.
There is a typographic error in my reply that might not be important, except that some forum users hang on every word: I should have written that "most" or "almost all" sexually active persons acquire HPV, not "all".
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