Statisticians at the University of Salford have developed a model of the spread of Aids which questions official estimates of its future in the United Kingdom. If correct, the findings indicate that reported Aids cases will not reach their peak until next year, contrary to official projections which suggest the peak has already occurred.

According to the new evidence about 1,100-1,200 cases of Aids in homosexuals and bisexuals will be reported annually over the next three to four years. This peak of incidence is "at variance" with official estimates, say Brian Dangerfield and Carole Roberts of Salford's Centre for Operational Research and Applied Statistics.

The researchers say that new cases of Aids among British homosexuals could be significantly higher by the turn of the century than official figures suggest.

Official estimates show a peak of HIV infection in 1983/84, compared to the new research which puts the peak later, in 1987.

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The Salford model of the spread of Aids involves transmission among male homosexuals in five countries namely the UK, France, Germany, The Netherlands and Switzerland. It shows a rising trend in Aids among that group in all European countries. The research- ers say the discrepancy with official estimates merits further research.

The Operational Research Society is publishing a collection of related work on HIV/Aids edited by the two Salford statisticians. One paper, based on research by mathematicians at University of Wales College of Cardiff, shows a consistent pattern of increasing high-risk sexual practices from 1990. The predicted change in the course of the epidemic is described as "both interesting and alarming".

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"Responsible attitudes" shown by homosexuals during the mid and late 1980s resulted in the Aids epidemic failing to reach predicted levels. But the new findings indicate that a relaxation in homosexual lifestyles "could well change the course of the epidemic in a disastrous fashion".

One indicator, the incidence of other sexually transmitted disease among homosexual men, shows a sharp rise from 1989. Moreover, say the researchers, a recent study collating information on rectal gonorrhea, hepatitis B, newly diagnosed HIV and syphilis, all indicated increasing high-risk sexual practices from 1990 onwards.