It is useful to consider specifically the impact of undiagnosed primary infection. Primary infection refers to the first few months of infection, when individuals have exceptionally high viral loads in blood and in genital fluids. It is estimated that the per-act risk of HIV transmission is 26 times greater during the first three months of infection than during chronic infection.

By definition, a person with primary HIV infection has recently engaged in a behaviour that led him or her to acquire HIV, and that behaviour may be continuing. However, because primary infection only lasts a few months, there may be fewer opportunities for transmission than during several years of chronic infection.

It is difficult to diagnose individuals during primary infection. All HIV diagnostic tests have a ‘window period’ during which they cannot detect a recently acquired infection. While most individuals have symptoms of HIV seroconversion, these may be mistaken for the ‘flu or glandular fever by doctors. People at risk of HIV  infection are often unaware that a sore throat, rash and fever can be symptoms of HIV seroconversion (especially if the symptoms occur together) and may not seek health care.

Increasing the frequency of HIV testing is likely to increase the number of infections diagnosed early (see How often should people test?).

A study of London gay men diagnosed during primary infection found considerable behaviour change in the three months following diagnosis. The number always using condoms during insertive anal intercourse increased from 31 to 61%; two-thirds reduced partner numbers; and there were far fewer diagnoses of sexually transmitted infections. The researchers argue that even short-term behaviour change at this time of increased infectivity is likely to prevent new HIV infections.