These can be done in individuals whose characteristics are known and can be controlled for, and if the relationship truly is monogamous then infections by acute STIs and from outsiders can be ruled out.
In these circumstances, it is easy to see why condoms sometimes fail, even in consistent users.
In addition, however, people are not consistent in their use of condoms, and may not even be consistent when they claim to be, or think they are.
However, because they are not always used correctly even if they are used consistently, studies have found efficacy rates of 85 to 87% when young women use condoms as their sole form of contraception.
Condoms are, however, the only method on that list that has been shown to protect against STIs as well as pregnancy.
Another is that the HIV-positive partner will be chronically infected and so will not have the very high viral load characteristic of acute HIV infection.
Thirdly, in long-term serodiscordant relationships, studies have shown that the HIV-negative partner can acquire a degree of immunity to their partner’s HIV.
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.
One widely quoted remark of this nature came from Ugandan President Yoweri Museveni who, at the Fifteenth International AIDS Conference in Bangkok in 2004, advocated for HIV prevention based on “optimal relationships based on love and trust instead of institutionalised mistrust, which is what the condom is all about…I think of condoms as an improvisation, not a solution”.