Men and women who were infected heterosexually were least likely to know they had been infected. The Aids diagnosis was also significant: adolescents and men over 50 were largely ignorant of their HIV status.
Most of the people in the study group had carried the virus for several years, having been infected in the early 1980s.
Professor Ian Weller, of the Middlesex Hospital, London, told the International Aids Conference in Berlin yesterday that the study had worrying implications for the spread of HIV in all groups.
Those infected were missing out on early treatment of the infections associated with Aids which could prolong life and on counselling and support.
Kholoud Porter and colleagues, from the Public Health Laboratory Service, examined reports of 3,556 adults with Aids for whom the dates of the first HIV test and Aids diagnosis were known, between January 1989 and December 1992.
This comprises a substantial proportion of the known Aids cases in the UK; 7,460 cases had been reported to the end of April. The researchers found that 49 per cent of the Aids patients learnt that they were infected within nine months of being diagnosed. More than three-quarters of heterosexuals infected were unaware before this time.
As expected, people who lived outside the Thames area were less likely to know. In 47 per cent of the cases (2,674) reported in the Thames area, the patient was unaware compared with 54 per cent (of a total 882) of reports made outside.
Lack of awareness among infected gay men (2,746) was greater than expected, but more striking still was the 78 per cent of heterosexuals (474) who were unaware. Women and non-whites were less likely to know of their condition. The year of diagnosis was also found to be significant - those diagnosed in 1992 were least likely to know. The researchers said more work was needed to identify and target this group for health education and treatment purposes.
Condoms and needle-exchange programmes and bleach to clean injecting equipment should be available to prisoners to limit the spread of HIV, the World Health Organisation, said yesterday.
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