Epidemiology and HIV’s Physical Properties

OCT.19 '95 7:32AM 310-446-1664 LAYNE @ UCLA. [phone] P.2
COMMENTARY
Epidemiology and HIV's physical properties
Scott P. Layne
Epidemiologic studies indicate that HIV strains with increased transmissibility are emerging. Systemic
surveys should be initiated to determine which viral characteristics govern transmissibility and to Identify
strains that pose the greatest threat to public health.
TUE spread of human immunodeficiency virus (IIIV) depends on the behaviors and interactions of
people as well as the inherent transmissibility of the virus. In developed countries, epidemiologic
studies show that the risk of person-to-person transmission ranges from one to five new infections
per 1000 sexual encounterst. In developing countries like Thailand, more recent epidemiologic
studies suggest that the risk of transmission is one order of magnitude greater - ten to fifty new
infections per 1000 encounters2. Attempts to attribute this increased transmission to lknown HIV
risk factors such as the numbers of sexual partners, frequency of encounters, varieties of behavior, o
amounts of drug use, and prevalence of sexually transmitted diseases have revealed no clear -
connections. We are thus confronted with the worrisome possibility that certain IIIV isolates are
appearing which are more transmissible than others. This is accompanied by reports of a rapidly
growing HIV epidemic in Thailand, which now appears to involve viral isolates that belong to
particular genetic subtypes3.
We are facing a precarious worldwide situation. There are no vaccines to prevent the spread
of HIV infection and their timetable for development remains uncertain. Antiviral drugs (such as
AZT and the latest protcase inhibitors) may help to prolong the lives of infected individuals but
they have no impact on preventing new infections. Much progress has been made in identifying
how HIV infection spreads (sexual intercourse, mother to child, needle sharing by drug users and
blood transfusions) and launching public health programs to reduce these modes of spread4. The

important strides we have made thus far in controlling the epidemic, however, ultimately may be
limited by HIV's apparent ability to increase its transmissibility.
HIV was first isolated over twelve years ago and since that discovery, we have made
unprecedented progress in dissecting its "molecular-genetic" properties. We have identified and