Studies of condom efficacy have therefore largely contrasted HIV and STI incidence or prevalence in people who claim 100% consistent use against people who use them inconsistently or not at all.

Because these studies involve private behaviours that investigators cannot observe directly, it is difficult to determine accurately whether an individual is a condom user and whether condoms are used consistently and correctly.

The efficacy of an intervention is how well it works in a scientific trial or when people use it as indicated, i.e.

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.

Another kind of study is to conduct a prospective cohort study, looking at differences in HIV incidence between two groups of people according to their usage of condoms.

This cannot be a randomised controlled study, but participants’ HIV and STI incidence can be related to their condom use either at baseline or preferably (because behaviours change over time) by means of regular questionnaires and monitoring.

Laboratory studies and product testing have shown that reputable condoms tested in the laboratory are completely impermeable to micro-organisms as small as viruses.

However, the same studies show that condoms come off the penis altogether 3 to 5% of the time but may slip down (but not off) up to 13% of the time.

The evidence we have is based on three types of trials, and each has potential weaknesses.