Do condoms work?

The ability of condoms to stop HIV has sometimes been questioned by people opposed to their use on religious or moral grounds.  Therefore questions of condom efficacy have to be addressed.

In one of the most highly publicised statements, in October 2003, the President of the Vatican's Pontifical Council for the Family, Cardinal Alfonso Lopez Trujillo, said: "The AIDS virus is roughly 450 times smaller than the spermatozoon. The spermatozoon can easily pass through the 'net' that is formed by the condom. These margins of uncertainty...should represent an obligation on the part of the health ministries and all these campaigns to act in the same way as they do with regard to cigarettes, which they state to be a danger."¹

These statements are quite simply untrue. Consistently used, condoms provide significant protection against HIV, pregnancy and sexually transmitted infections (STIs). The degree of protection they offer against HIV and STIs is significantly better than any other single prevention method, taken in isolation, other than sexual abstinence or complete mutual monogamy between two people who have tested negative for HIV.

Studies, described below, show that condoms used 100% of the time, though not necessarily 100% perfectly (i.e. with usual rates of breakage, slippage and so on) provide protection of about 80 to 85% against HIV (uncertainty range: 76 to 93%). In other words for every 100 cases of HIV infection that would happen without condom use, about 15 (range: seven to 24) would happen when condoms are used consistently.

Finding out the degree to which condoms protect against other STIs is important for sexual health in general and may be particularly important for people with HIV, who may be more vulnerable to the effects of certain STIs. Studies^{2 3} also show that:

• Condoms offer a similar degree of protection against gonorrhoea as they provide against HIV.
• They offer protection in the range of about 50 to 66% against syphilis, though this depends on factors such as the location of primary syphilis lesions.
• They provide protection against chlamydia and trichomoniasis of a similar or somewhat smaller degree, with different studies showing protection rates varying from 85 to 26%.
• Against genital herpes (HSV-2), estimates of efficacy range considerably; studies are hampered by the fact that people with herpes are only intermittently symptomatic and/or infectious. The best estimate we have is that using condoms more than three-quarters of the time halves the chance of acquiring HSV-2, and may reduce the chances of genital infection with the cold sore virus HSV-1 too.
• One study has demonstrated that consistent condom use offers women significant protection against HPV infection by men (in the region of 73%). Another has found that condom use helps to prevent HPV infection progressing to cervical or penile cancer in both women and men.