A group of men who may be underestimating their HIV risk has been identified in a new study by the University of Edinburgh.
Public health messages should be targeted specifically at the neglected group - men who have sex with other men but who are not openly gay, researchers said.
Earlier work from the same group suggested 6 per cent of men claiming to be heterosexual at the time of diagnosis had actually become infected through sex with men.
They tend to mix with, and acquire infection from each other, not from openly gay men.
They are unlikely to mix in the same social venues as openly gay men and are not likely to disclose they have sex with other men. Fear of stigmatisation, rejection or prejudice can stop this group, who include bisexual and non-gay-identified men, from disclosing their sexuality.
They are also less likely to receive prevention messages and access the same healthcare as others and as a result may be less aware of their HIV risk.
Researchers used a national archive of anonymous data to study patterns of HIV transmission.
They analysed the genetic code of virus samples from more than 60,000 HIV-positive people in the UK.
Because the genetic information of the virus changes rapidly over time, by finding people whose virus was more similar, scientists created networks of linked infections to see how the virus had spread.
This study found the group of men identified tend to have fewer sex partners and prefer to partner with each other - behaviour that may lead to them underestimating risk.
Men who have sex with men are the group most at risk from HIV and account for half of those with the virus in the UK, but they tend to be diagnosed and receive treatment at earlier stages of the disease.
In contrast, heterosexual males remain the group least likely to visit sexual health clinics and are often diagnosed late, when their immune system has already been damaged.
The study, published in The Lancet HIV, was funded by the United States National Institutes of Health.
Professor Andrew Leigh Brown, of the university’s school of biological sciences, who led the research, said: “Nondisclosed men who have sex with men are more likely to be infected by each other than by openly gay men, and less likely to be aware of their risk.
“The finding shows public health messages should be targeted specifically at this neglected group.
“It also shows large-scale studies of health data can be carried out without risk to individual privacy.”
Figures from HIV Scotland show 5134 people are diagnosed as living with HIV. Thirteen per cent of those living with the condition are unaware of their status.